The reason many places don't do this is not so much to avoid feeling like an a-hole but to avoid accidental liability. There are so many things you can't say or ask in an interview, that a lot of companies that are big enough to have an HR person or department will require a very generic rejection letter just to be sure they don't provide grounds for a lawsuit.
Is there a way I can just waive my ability to sue? I would love to know my weaknesses in the eyes of others.
In other words, your signature does not provide a lot of legal security