Why does ppi matter for web images?

Question

Hello
When placing an image in my (web/pixel) project the resolution is dependent from the set ppi.
When I create a new document for web, then logically only the pixels matter. Nonetheless there's a ppi field, why? And depending what value it has, placed pictures (file > place or drag&drop from Windows' explorer) get resized instead of the wanted 1:1 resolution.
1 pixel should remain 1 pixel when working with in pixels.
Try it out:
Create a new document with Full HD resolution (1920x1080 pixels), set ppi to 10.
Import a Full HD picture (Blu-Ray screenshot whatever) and it's mini-sized.
My screen resolution is 59 ppi (I use my 39" TV as monitor) and this is set in the Photoshop preferences (in case I do some print stuff that I get a 1:1 view if wanted). If there was any logic behind the pixel-ppi-placing thing, then this set standard monitor ppi should bring me a 100% sized picture... but it doesn't. Only when the ppi is set to the fantasy value of 72 I get a 100% sized placing.
This can be disturbing and is very annoying when starting a new project by opening an image that does not come with 72 ppi by default (my Canon camera makes 180 ppi JPEG photos)  and placing new pictures in this project. They get opened in wrong sizes, in my case largely upscaled, with no way to correct it but guess-scaling it down.
Bug?
Pls pls fix it.
Ppi has nothing lost in pixels-only projects.
The only way that works is to open all single images in Photoshop as tabs, and then drag&drop from within Ps. -.-
Regards
Mopsi
Example screenshot: http://www.m-i-u.de/images-i83580bxvogj.jpg
The yellow framed layer is a image from my camera, opened in Photoshop CS6 extended. 3264x2448 pixels (180 ppi).
The green framed layer is a screenshot from a movie, drag&drop from the explorer. 1920x1080 (normally) unwanted upscaled here.
The red framed layer is a screensot as well, but drag&dropped from a tab within Photoshop. It remains in its original resolution of 1920x1080.

Answer 1

I'm wrong and your right. I just did some testing and Photoshop does indeed interpolate the lower resolution 600x400 72DPI image is  up size to match the higher resolution document document size during the place process. I just assumed Photoshop would preserve image quality and not interpolate the image.    As you have shown it does interpolate the image which greatly lowers the image quality of re-sized low resolution image.
This shows you should not use Photoshop to merge images into a composite if they have greatly different resolutions.  I don't have a problem there for I never use "save for web" to save images to be displayed on displays from the web or my machine.  I use Fit Image and save as, or a Image Processor script that uses Fit Image to re-size and uses save as.  The leave all my images files resolution setting unchanged.   For some reason Save for Web changes all jpeg files it saves resolution  setting to 72 DPI even if they are 8MP images for high resolution displays..
During testing I also tried using Photoshop's script Load Files into Stack instead of using Place.  That script works the way I assumed Photoshop Place would work.  Images layer are not interpolated they remain the correct number of pixels and the image quality is not is not changed.. The image layer are normal layers but can be converted to smart object layers however the object would be normal layer not an image file.
When you use Place you get
It also possible to undo Photoshop's Place scaling with a simple calculation.  Divide the image layer's original dpi by the document's dpi here there are three layer that did not start at 300dpi. two started out as 74dpi image file the other a 500dpi image file.  72/300=.24 = 24% the other 500/300=1.66666 = 166.66%
all you need do is change the associated layer's image transform width and height scale from 100% to the calculated percents
The only time I set document to 72DPI resolution is in Photoshop Scripts so I can calculate a font size  for a charter sting so it will fit the canvas size.  Photoshop Text Tools seem to be tied to 72DPI resolution.  Once I add the text layer I restore the document DPI resolution back to its original setting.
So when it comes to Photoshop all files use in a project should have identical DPI resolution for best result when making composites.    My image files are either RAW files which have no DPI for they are not RGB image files. Or RGB Image files that have a high DPI resolution. I process images for print. Image files DPI resolution is meaningless when if comes to Display Screens.  All that matters is the number of pixels a display can display and the number of pixels in the image. Even images I re-size for display screens DPI are high for I do not use "Save for Web". I see no good reason to strip metadata and change resolution to 72DPI.  I tend not to interpolate image except the ones I save to be displayed on a display.  I change Print size by changing the DPI setting without resample.  My Epson 4800 inkjet printer has no problem printing high resolution pixel I see no reason resample my camera 16MP image down in size just to print at 300dpi.  If I want to a single 6"x4" print I see nothing wrong printing it at 816DPI.  My eyes can not resolve down there the printer can.  However when Printing on Roll Paper many 6"x4"  the composite document I create via a script has a 300DPI resolution and the image layers are resized to be 6" x 4".