Yes, that's among the definitions for amateur, but the first definition is "a person who engages in a pursuit, especially a sport, on an unpaid rather than a professional basis."
Pilots using ATC services range from the 15 hour student pilot to the 150 hour private pilot like me to the 15,000 hour airline pilot. No, we're not all professionals, and ATC is required to work with that entire range. Furthermore, "preferred" phrasing from the AIM is just that - preferred, not required. Plain English is perfectly acceptable, and with a sky full of non-professionals it is unreasonable to expect everyone to talk like the captain of an airliner. It's also to be expected that jargon and slang will work their way into communications, and I'd be surprised to learn that ATC has a serious problem with it.
For example, I usually respond to "Frequency change approved" by replying, "10-4, good buddy. Threes and eights and all them good numbers to ya, and we'll catch ya on the flip. Buh-bye!"
