I use to change my birthdate on a few social networs every few months and I would get always the same people sending best wishes and such.

Would you have preferred that they wrote down everyone's birthday, or not have wished you their birthday? I don't see how this applies to the LinkedIn situation.

I always assume birthdays on social networks are lies like mine is and never wish happy birthday or I ask in person.

I'm pretty sure I've sent those kind of misguided congratulations, probably twice that I can remember. One was a guy who I knew had moved to India, so I congratulated him on moving back to the U.S. - I think he had really just changed the status of his online consultancy. The other was just updating the name of the company because it had just been bought out. I never got a response from either of them.