A rudimentary explanation of supply and demand:
A plane only has so many seats, the first seats go cheaply because of low demand and high supply.
The last seats go expensively because of low supply and high demand.

Only UA has full access to the full supply of seats (100 for arguments sake).
UA opens up a portion of those seats to partners, in this example case AC gets 10 seats they can sell "as their own".
AC sets pricing based on their restricted supply.

AC sells 9 seats.
UA sells 0 seats.
UA may have 91 seats available inside that specific plane, but AC only has 1.