I think this is the result of an A/B testing article which showed higher follow rates for "You should follow me" than "If you feel like following me" and other less imperative wordings.
(Is it really rude? I dunno; it doesn't offend me when I see it, though it does grate a little. But when someone says "X is rude" it seems pretty weird to respond by saying "There is some evidence that X achieves something the person doing it wants", as if that were actually responsive to the complaint.)
Incidentally, I can't help suspecting that with more people using the "You should ..." form, its effectiveness relative to other ways of asking to be followed might be less these days.