All my icons have something done to them. I change the coloring to them, even if I don't have the borders/textures on them.
Every single icon I used on my old Blaze blog (when I began making 100x100)/every icon on my public icon blog HAS a different coloration from the original source AND it has different filters on them to make them look better in a 100x100 format. It's not like I took a few minutes on each image cropping them. If that was the case, it might be a little different for me. But each time I make a pack to put on my icon blog, it can take me UP TO AN HOUR or more to come up with a good-looking psd for that pack.
The reason people get mad when people take rather than ask is for that reason. A lot of people put WAY MORE EFFORT into cropping/editing than a lot of people realize. It can be hard to tell sometimes -- but the work there is done.
Here's a comparison to the icon you just used vs the one I made of the same image a few months ago.
Mine is much lighter, has a filter on it that sharpened the image, it has a white-to-blue gradient on it that makes the image pop out a bit more. and on a LOT of icons I make i take out unnecessary parts of the image.
in this one, i had to erase the text in the bubble AND some stuff in the background to make it all white. like there's a LOT of work that goes into these. and people CAN'T really stop others from taking things from an official source, you're right. even if these WEREN'T public icons I made, I can't REALLY sit there and demand that people don't take.
I can ask NICELY, but that's not going to stop people. It's much harder for people to justify taking icons with fancy textures/borders that are completely noticeable.
But that's why it's rude, imo. Because it's MUCH more for most editors than just cropping an image. And there ARE SOURCES of public, nice-looking icons. You just have to search for them but MOST PEOPLE are lazy and take without reading the rules of the blog and seeing if that's okay.