I don't have a huge monitor (22 inches) but I found it difficult to read because the lines were so wide. They should set a fixed width for the content and then I'd agree with you.
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.
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.
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.