Professional means you teach (notice the word root in "profess" as in "professor"). It really means you know enough that you can teach others how to do it right, not about get paid for it per se.
You have the etymology of "professor" and "professional" completely wrong. You can't just notice the same root in two words and then completely reinvent the meaning of one to make it have something to do with the meaning of the other. The evolution of language is complex. Here: http://lmgtfy.com/?q=etymology+profession
Do professional football players teach playing football? Some, probably, but not all. We don't call those who don't teach amateurs. They're getting paid. The amateurs are the high school and (arguably) college players, along with rec club and pick-up game players.
You're noticing a common root, but not the meaning of the word in the modern day.
Decimate means to destroy 1 in 10 of something (like an opposing army). But today we use the word to mean destruction of a large percentage.
I suppose an argument can be made that modern use of amateur is more akin to what used to be novice. However, I'd have a hard time accepting that except when it's used as a slur. We talk about amateurs in many fields, but don't intend to dismiss them as unskilled or inexperienced, we're classifying them as non-professionals. In a forum like this, filled with amateur programmers, it seems, to me, that it's wrong to misuse the term in this manner when a large portion of the readers here are amateur programmers but of moderate to high skill level.
