Question
Are US-based airlines mandated to comply with ADA guidelines and if so, to what degree?
Do they have to provide wheelchair accessible restrooms, ramps, etc. on board the aircraft? I understand different airlines have different policies but I am wondering if they are mandated to comply, what the minimum requirements are.

I was unable to book a flight for my disabled mother recently due to accessibility issues. She has a power wheelchair and I was told by one carrier I would be charged an outrageous amount because the wheelchair is considered "cargo."

Also, she is on oxygen and I was told I would have to pay an extra $100 for that as well as purchase another seat because she is a large person.  

Answer
I am not a compliance expert or a lawyer in that field. As far as I know, they must make 'reasonable accomodation', and they generally do a decent job.

Power chairs cannot fit on most airlines, but the airline will assist you in packing it and providing help with a manual chair and transfers (in most situations).

There are extra charges for this (as for oxygen, which is an incredibly dangerous material on a plane, and for an extra seat- which she can usually be reimbursed for if the plane is not full), but the charges are based on things they need to do to accomodate the situation- not a penalty or fee for annoyances, which would be improper.

So, as far as I know- this is irritating, but within the law.