One thing people seem to forget is that a fake address will make it harder to reply for people who would like to send you information that you actually want. This is assumed to be uncorrectable by a program. Thus the intention is to force the reader to check the validity of the address before he replies and to eventually correct it. A correct return address would make this unnecessary and this is how it was meant to be. Thus people overlook this, some simply don't care.
Some people put their correct address into their signature.
I think it should not be necessary to force a replyer to search the whole post for the correct address and insert it manually before replying. A correct address makes this superfluous. It is unfair to people who put good intention into their reply in order to help. Those who put this extra work onto others do not deserve to receive an answer to their question. Fake return addresses are used by spammers who do not wish to be mailbombed for their spams.