I personally don't like really wide signatures because they tend to be very distracting. Like, your signature when widened to take up the whole post space looks like a header and that would be really weird to me in the signature space. But that's personal preference.
My biggest issue with 100% signatures used to be that people would make the whole thing clickable and when you're switching between windows on your monitor you'd accidentally click anywhere in the area of their signature and end up clicking a link and it would be so annoying but... so long as that wasn't allowed, I don't see why it would be a big problem.
It's also just easier to police signatures when there's a strict limit on physical size. When the limit is 600px, you can usually tell at a glance if someone's breaking it or not. If all sigs were allowed to take up 100% of the horizontal screen space, it would be much harder to draw the line on what kinds of images are allowed. If someone's allowed to use THAT much space, instead of restricting their BG images to ~600x300, they could make them wallpaper-sized to accommodate most screens and use that as their sig image. Sure, most people would probably just tile a background image, or even no image, but it would be much more tempting to not do that and staff would have to check every time to make sure the file size is within the allowed constraints. Large images, even if simple in design, come with big file sizes which is definitely something I would never want in signatures. It's awful for people browsing on a bandwidth limit, which is most people on a mobile platform and plenty of people in more rural areas.