To answer the question, yes, AA does hold seats back, some pending assignment at the airport. Their seat maps are notorious for being inaccurate with assigned seats continuing to show as unassigned. This leads you to believe the flight load is lighter than actual and may keep you hoping for an upgrade even though all seats may be sold.

One event is not enough to demonstrate deceptive practices, but I'd say if it happens repeatedly, then you may be onto something. And I wouldn't be surprised if they are. They've been playing with the numbers for quite some time and I'm sure there is some model out there. In the meantime, if you just have to have that isle/window seat, pay for it. I do believe they all come available the day of or at least once the door closes. So you could risk not being able to move.