This is one thing that really 'grinds my gears' about AA. You do them a favor by altering your travel plans because they overbooked the flight, and sure, you get a voucher, but unless there's some incentive they're not going to knock anyone off the flight. Then they decide to charge you $8-15 depending on how you use the voucher because they haven't made a way to use vouchers on their website.
In my opinion, if they can't find a way to make vouchers usable on aa.com they shouldn't penalize their customers for doing them a favor.
If AA would fix this along with returning to show fares including taxes up front (at
least make it an option!) I'd be a happy man. It's the little things, I guess...