I think it's a bit hyperbolic to call it half the web. Only a small handful of sites cap password lengths. They might happen to be sites you use, but it's not nearly as common a practice as you seem to think.
I don't think you're correct. Anyone who's storing a password in plaintext is probably going to use a fixed-width field to do so. I'd bet half the internet stores plaintext passwords. A lot of the web is one-off e-commerce systems that no one should trust anything with.