They still have to check the car in, deal with the customer, put the car on an alignment rack, get out all the gear, give you a print out etc.. The hassle alone probably doesn't make it worth it for them to save you $30 by only doing the front/rear, especially because you might come back and complain that it doesn't drive straight or something afterward. That said, I think I had a front wheel alignment-only done on my girlfriend's beater years ago at a Firestone. Many shops would do a front wheel alignment but I haven't heard of someone doing a rear only. We don't mess w/ alignments so I'm not sure.