It's a worthy question... Technically, it is sort of illegal in a very gray area way... More like frowned upon by the banks... Messing with coins is a felony, since they used to be made of gold... Bills are only a misdemeanor, and not one case has ever been prosecuted... Intent to decirculate must be established, and I never even sign these...

Also, a precedent has been set by the culture around us... When Leonard Nimoy died, millions of Canadians turned their money into Spock dollars, as a tribute farewell... It became such a huge trend that the Canadian government had to issue a press statement requesting its own citizens to stop spocking its money!!

Finally, you know FIAT money is fake af, right? Federal Reserve is as official as Federal Express... The USD is backed by bombs... And that is sort of the whole point, makes me do what I do! The name Occupy Wallets is more than a good pun... More like, I'll just sit here in your wallet until you figure out what to do with me!