Users value content. That's what they visit the webpage for. However, businesses like this (i.e. the ones inventing ideas like sign-up overlays) do not care about the content. They might say they do, but that is a lie. Website content is only a means to an end - extracting money from users. It's a bait for the fish.
This fundamental mismatch of values is - I think - the main reason why people still can't get why websites implement annoying sign-up pop-ups and stuff. Such companies don't really want to provide value to their users, they only pretend they want, to the extent that maximizes profit.
I'm fine with people earning money and charging for their work. But I believe that this relationship should be up-front. Running heavy maths to figure out the optimal amount of pretending-we-care to maximize profits is not only annoying, but dishonest.
I wish there was a way to reward businesses that actually focus on benefit to their customers, while punishing the ones who only pretend. Right now the only thing I can do, whenever I see annoying sign-up box or other signs of someone doing "clever things" to extract money from me, is to say "fuck you, I'm not coming back".
As for the usability part, [0] summed it up perfectly:
"(...) you have no fucking idea what a website is. All you have ever seen are shitty skeuomorphic bastardizations of what should be text communicating a fucking message."