It's not so much the airline itself as it is the baggage check service outside of he terminal that you are presented with after debarkjng from the rental car shuttle. I checked my bag and after paying for the bag the attendant launched into this spiel about "this is a free service provided by United Airlines and blah blah blah" only to conclude with "tips are accepted." First off, I am NOT tipping for a service that has to be provided for me. I HAD to check the bag. I'm not going to tip you after paying $25 to check my bag. Second, if that is the case, they need to provide some sort of sign or something indicating that it is a tipping service so that the awkward "non-tip" scenario can be avoided. Not that I feel awkward however the attendant became disgruntled after I walked off and treated the person behind me like crap! United needs to rethink this situation and above all, avoid the outdoor bag check service if you don't want to tip after forking out money to check your bag.