Probably because they considered what you were doing as Spamming. If you created additional IDs, then they could legally consider that hacking. Both are valid reasons to contact your ISP about your behavior. They were nice enough to give you a warning not to do it, probably an idea hatched by their lawyers.
I don't see how they were heavy handed as you put it, since you were warned and still did it (multiple times at that). What did you honestly expect?
It might be wise to read the terms of service agreements or board rules for sites and forums you visit, making sure you understand them. In fact, you might want to go ahead and look over MacOSX.com's
Board Rules while you're at it.