The same reason they defend the drunks, the delinquents and the piss poor pilots that can't fly....so when YOU ask "what am I getting for my 2%" they can say "look what we did for these folks"
It sounds like ALPA is getting tagged with the misperception that they "defend" intentional malfeasance. In the very few cases that I have heard about (ALPA keeps good secrets, as they should, but sometimes the pilots talk) ALPA has negotiated "graceful exits" so the pilot can at least gain other employment elsewhere. Fired is fired, but "quit" allows a person to go get a job (likely a non flying job) where a PRIA report will not find them.
ALPA is concerned about safety too. ALPA works with the FAA to enance safety. I think the Runway Incursion Information & Awareness program was an excellent example of the way things are supposed to work to make our flying a safer operation for our passengers, we pilots and our employers.
...and then there are the cases where airlines just make up a load of crap to tag a pilot with an incident. Lets discuss that because it happens in this business at every airline.
There are two Certificates that allow your aircraft to fly. Yours and your Company's. If an incident gets investigated by the FAA the Company will never admit it was their fault and take the penalty on the Company's Certificate. The Company will find a way to pin it on the pilot - and most of the time the pilot was the person who had final authority over the operation.
It is no mistake that the responsibilities of the VP of Flight Operations is about one short paragraph long, while the responsibilities of the Pilot in Command go on for 40, or more, pages.
ALPA can provide some useful help because they have good prior experience dealing with these messes, while most pilots have never been in trouble before and will never get in trouble again. When (or if) you get in trouble, it is a benefit to be able to talk to someone who has experience with how the whole thing works.