About the two Saudi men having a valid rationale for flying, there has been a trend over the past few years of al-qaeda using "Tourism Dry-Runs" to check reaction times, find weaknesses etc.
A recruit with a valid excuse for travelling will act very suspicious on planes, at vulnerable sites etc. just to see what they can get away with. There are stories of several Middle Eastern men rushing the cock-pit, then backing-off and then standing around on the plane at the exits whispering to each other(Syrian musical group with a booking in San Diego). Middle Eastern men driving around in a van asking everyone they come across the question of where is the nuclear plant (when stopped, they having fishing gear and they say they just wanted to go fishing at the nuclear power plant, even though it isn't fishing season.) Suitcases left unattended at the subway, etc. etc. There are hundreds of examples.
When these Dry-Run Tourists are released (there is nothing to charge them with), they report back what happened, especially if noone stopped them.
Sounds like these guys may have been testing the watchlist reaction timing.