There are quite a lot of people in this thread who sound like AT&T employees trying to add to the scare tactics.
The cheapest and most efficient way for AT&T to "detect" tethering would be to monitor data usage and accuse high data users. They anticipate a lot of them will be uneducated and believe they have really been caught and switch to the official tethering plan (losing their unlimited, which I believe is AT&T ultimate goal here), or will simply ignore the text and they can automatically switch them, accomplishing the same thing.
If AT&T accuses you, and you refuse to switch to the new plan and claim you're not tethering, and they switch you anyway, you'd almost certainly be entitled to cancel with no ETF, legally at least. Particularly if they refused to do anything more than say "we suspect you are tethering" without providing any support.
To be fair, AT&T contract does say they can modify or terminate your account if the simply believe you are tethering, but no court would hold that belief, without a legitimate basis, is grounds for modification or termination, and it's hard to believe that 20 gb of data usage in a month would be a legitimate basis for that belief (those who are reaching ridiculous numbers like 50+ might be a different story.
My take (law student with no tech background): if they accuse you and send you the message, call them and play dumb and say you do a lot of streaming. If they buy it, great. If they end up switching you anyway, or forcing you to switch, you can presumably get out of the contract with no ETF. If this fails, and you have money to blow to prove a point, you can probably seek an injunction preventing AT&T from altering your contract, or a declaratory judgment that the contract permits you to get out of it without an ETF in this circumstance.
Odds are that AT&T would be unlikely to show up for any lawsuit filed by an individual over a few hundred bucks, which would entitle you to both the ETF and your legal fees.
Granted, I'm a student not yet a practitioner, so all of this should be taken with several grains of salt. Additionally, none of this should be construed to constitute legal advice.