It's a roll of the dice, they do a very extensive secondary inspection on some people, and it isn't always based on suspicion, sometimes it's just random. If you have a very small amount, they might just keep it, they might fine you, and they might charge you. The more you have the better the chance they will charge you. It all depends on the customs agent they make up the rules based on guidelines they are given. Remember, when you deal with US customs, you aren't in the USA yet, you are still technically on "international ground" (yes even inside the states, there is a zone before you get through customs where you are still technically not in the country) so you don't have the same rights as a US citizen. And some countries (like Australia) are very much looking for AAS.
You are correct, the chance of getting caught with a very small amount of tabs is slim but why risk it when the penalty can be so extreme?