Originally Posted by
halls120
So, if I buy a ticket on UA, and they can’t fly me to my destination on UA metal and book me on a different airline, and it costs more to fly on the other airline, it isn’t UA’s responsibility to pay the increased cost?
Well, yes and no.
OP was offered transportation at no charge... and if the bags had already been in UA's possession, they would have transferred them to AA (or, at least, they would have claimed that they would have done so). OP could probably have forced the issue by trying to check the bags in anyway onto the original flight.
Once OP accepted a re-routing and carried the bags to AA, the conditions of the underlying fare apply once again, and OP was entitled to the non-status passenger's allowance.
Now, that said, I would write back, include the receipt, and ask for an ETC to cover it.
Also, why was the BUF-ORD cancelled? If it was weather, UA has a reasonable argument that they actually went beyond their responsibility to put you onto AA in the first place (although, as a 1K myself, I'd expect them to do that).