I have done this a few times for some first questions, and I don't do this out of "fairness", it is my assessment whether this first post deserves standing at $-1$ in my opinion. And I can guarantee you that "-1" has quite a different psychological impact on a new poster than "0" and a comment.
I don't see how one or two +3s for the very first post of a new user is a problem or will lead to more borderline content. It is in fact quite unlikely that a new user understands how they got +3.
I also sometimes happen to upvote first and non-first posts who stand at -1 because I just want to upvote.
I would say that this phenomenon is a consequence of the system and I actually think that is very unlikely that people are smart enough to hit the right obfuscation level on this forum and at the same time want to exploit this to gain reputation. (Except maybe one time for curiosity's sake.)
I disagree that voting "should" be independent of other votes, but this may be a topic for a different thread and "should" should be defined first, although I probably disagree for most definitions.
(Simple recent example: I happened on an older thread with a highly upvoted accepted answer where I added my vote. Then I read the second answer with significantly less votes and realized that it was better. So I removed my vote from the accepted answer and upvoted the second because this was the only way to help indicate that the second answer was better, especially as many people would not read the second less-voted unaccepted answer. Now, the accepted answer totally deserved my upvote, so my vote was based only on the voting pattern.)
Edited to add: The example shows that the votes of the people who "only vote on the content" also depend on prior votes. Since everything is sorted by vote, it is wrong to pretend that there are any "independent" votes.