Delta: We limit the number of personal wheelchairs to one personal wheelchair per flight...Wheelchairs stowed onboard have priority over other carry-on items except those of through passengers.
US Air; Manual wheelchairs, canes, crutches and walkers
On aircraft with more than 100 seats, US Airways has room for one typical adult-sided folding, collapsible or break-down manual wheelchair, cane, crutches or walker in the cabin’s priority stowage space, so long as it complies with applicable safety, security and hazardous materials rules. The priority stowage space will either be a closet or the back row of seats.
US Airways offers in-cabin wheelchair stowage on a first-come, first-served basis.
Please call 800-428-4322/TTY 800-245-2966 24 hours before your flight to document your request for in-cabin wheelchair stowage.
American: Wheelchairs and other reduced mobility assistance requests can be made while you are booking your flight on AA.com or by calling Reservations.
If advance notification is not given, the airport authorities will make reasonable efforts to assist you, however an advance notification will help ensure you receive the assistance you require.
I normally fly US Air and as noted above the single chair space is supposed to go to the one that reserves it first. Just showing up as the OP did is incorrect and they don't have to help you if they don't have room.
Pax with disabilities should be treated with no more special attention than a regular passenger as long as they are given the same abilities to fly. Delta specifically notes that wheel chairs do not have priority over other passengers.