a) Plainly have not taken your skills or location or pay grade into consideration
b) Refuse to take "no" for an answer
c) Turn hostile or somehow imply that you owe them your time or should be grateful for their attention
And we generally assume that the engineers are correct in their complaints. I can't say I've ever seen recruiters being abusive, but I have heard voicemail messages that start with "I know you said not to call you again, but...", and there's really no question about who is in the wrong there.
Whether it's actually important is a different question. I get a fair few pointless enquiries via LinkedIn, and I ignore them. I can't say that it troubles me much. But if the people sending those messages were insulting me, making sexually explicit statements or generally being persistent beyond all efforts to make them stop, I would probably feel much more strongly about it.
What puzzles me is that the behaviour of the recruiters does make at least some sense in a way that the dating site behaviour doesn't. Sure, getting a job spec for a senior Java developer and then mailing everyone with "senior [something] developer" in their past job titles on LinkedIn is going to fail quite often, as you'll hit Senior Python Developers and Senior C++ Developers and so on, but sooner or later the recruiter has to get lucky. But who the hell responds positively to the kinds of comments that the OP received? Are the people sending the messages doing so because they believe that sooner or later it's going to work? Even worse, does it actually work sometimes? I can't easily believe that it does, but it's hard to understand the behaviour otherwise.