No, you're not overreacting. It's called "vote/follow bartering", and it doesn't fly here very well. The people doing it are either new or bots. I'm betting a good blend of both.

Sending someone an unsolicited private message with no intention of being human to the person you're messaging is spam. It's no different than knocking on someone's front door, making them get up out of their comfy chair, only to throw a business card in their face and run away once they open the door.