They shouldn't. If you want to make a wide page narrower you can always resize your browser. If I want to make a narrow page wider, there's no way for me to do that.
The argument is the same, and they're both invalid. It takes way too much effort. I'm not resizing a window I use for everything just so I can fullscreen it when I switch to another tab and resize it again when I return.
The difference is that one is accessible to the average user. Outside of Developerland, most people don't even realize that the page they are viewing has a source which can be inspected or changed.
I'm pretty certain my mum, my daughter and pretty much everyone who's ever used a computer can do the former. I doubt that too many can do the latter.