I clicked through to the wikipedia entry for the first 10 detainees, in no case is possession of this particular model of Casio watch the only or even primary point of evidence in support of their detention. Some choice bits from various entries for people on this list:
The detainee met with and received money from Usama Bin Laden.
The detainee fought on the front lines for approximately nine months and fired his weapon at coalition forces.
Detainee stated he offered to help the Taliban.
The detainee resided in Taliban safehouses during his travel to and within Afghanistan.
This is certainly a curious thing, but without further info and context I have no basis to make a judgment one way or the other whether possession of this particular model of watch while in Afghanistan/Pakistan is sufficiently unusual to be used as a point of evidence in proving a close affiliation with al qaeda/the taliban. Unfortunately, this wiki page does absolutely nothing to further that investigation, other than to highlight something seemingly odd.
The list is surprisingly long. Obviously detaining someone simply because they wear the cheapest mass-market watch out there is ridiculous, but could it be statistically unusual?
It's a kiss-simple, cheap, innocuous (until recently) timing, team coordination, and secondary intelligence gathering mechanism (via time-indexed observation logs of potential target activities).
These models are probably available for cash (at or below cost) through a plethora of street-side vendors, pawn shops, and cheap electronics retailers in any metropolitan area.
In principle, this is no different than "burner" cell phones historically used by drug dealers (and featured prominently in season 3 of The Wire).
I can envision more well-funded groups using Flip cameras.
Muddy application of Bayes' rule here: From what I understand, they (with a capital T) determined P(watch | terrorist) to be a high probability; however, this doesn't necessarily mean that P(terrorist | watch) is also high.
http://www.amazon.com/Casio-F91W-1-Classic-Black-Digital/dp/...