Yes, ads cost money. That’s because they’re a fair trade: your money in exchange for somebody’s attention. What you’re trying to do is hijack people’s attention without paying for it. Which is offensive.
This exactly analogues my own thoughts on the subject. I think many companies are using social media in an ineffective and upsetting manner because somewhere down the line the concept behind Twitter (as a mechanism for conversation) became conflated with the concept behind a mailing list. Like Nic, I want to talk to people and not be talked at by a brand. The system only has value when all of its constituents recognize the conventions in force and the convention here is that we all take turns bringing value to the conversation.
P.S. You can actually talk to me on Twitter.