The flip side of that is, you can't refuse the USPS. Everything I care about comes to me electronically or through a parcel service, I only need USPS a couple of times a year (USG stuff like DL renewal). Even so, I need to collect my junk mail at least twice a week - because if I don't the box will fill up, they'll leave a threatening note about returning future mail to sender, and I'll have to pick up the stack at the local post office. I've done that a few times now. Its always funny how when I get there to pickup the volume of mail that was too large to fit in the mailbox - there are only a couple of official looking envelopes that turn out to be junk mail. The workers obviously recognize the fact that I don't want three copies of "The Penny Saver", and they don't want to waste space in the PO warehousing it. The incentives are way off in this system. They get paid to move junk mail and I am forced to accept it.

tldr; I hate junk mail and will scream into the void about it.

There are actually laws written to prevent anyone but the government delivering "letters or packets" under penalty of fine or seizure: http://www.law.cornell.edu/uscode/text/18/1696. I don't know recall details or the outcome, but I recall a few years ago it was up in the air whether you could send documents legally with private services, or if there were fundamental differences between USPS and "courier" services.