US Immigration and Customs Enforcement asks all US citizens where they've been when they return to the US. We fill out the same customs forms as non-US citizens, and these are reviewed by both the immigration agent and the customs agent.

Here in SFO (for example), it is quite common for single, male US citizens who have been in East Asia to be searched and questioned extensively. Quite a few individuals are arrested every year for bringing back illegal naughty pictures into the US. Same thing goes for US citizens returning from Amsterdam, the concern of course being pot.