They are the only government institution required to do this, and it has crippled their ability to remain profitable
The post office may be the only government institution required to do this, but every private institution is required to do it.
http://thomas.loc.gov/cgi-bin/bdquery/z?d109:HR00004:@@@L...;
Since the post office is supposed to be more or less independent of the government (i.e., profitable all by itself, rather than govt. subsidized), it's quite reasonable to demand that it be subject to the same pension regulations as the private sector.