I think airport police should be able to stop anyone suspicious.

A lot of people would agree with you.
The sworn testimony of the airport police who stopped Stephan was NOT that they suspected he was a terrorist, or wanted to check out suspicious behavior. The reason they stopped him was that they thought his bicycling on Outbound Roadway was dangerous for him and for motorists. Remember, at the time, there was no sign forbidding bicycling on Outbound Roadway. There was a "no pedestrians" sign.