Spam should be defined from the point of view of the user, not the company! I understand you, but I disagree... ;-)

If users don't like Twitter's monetization strategy, they can use an some other microblog or social networking service.

It's kind of like saying "I don't like paying for coffee, so it's OK for me to break into Starbucks at night and steal some." It's not, because the terms of the transaction aren't that. They're "if you give me money, I will give you coffee" and you can agree and trade money for coffee or disagree in entirety and go without the coffee. You aren't allowed to walk in, take the coffee, and quip "I'm altering the deal; pray I don't alter it any further."

Well, it's more like taking free coffee Starbucks are offering, then having them start putting it in cups with an advertising message on them and bitching that now you're getting gypped.

From the point of view of the user they opted in to twitter sending them sponsored messages and never opted in to your bulk unsolicited messages.

It's pretty cut and dry.