Pretty sure it's not an official plate. The sovereign citizens will print their own. Sometimes like this in an effort to fool cops into thinking its a regular plate so they don't get pulled over.
It's not a customized plate, it's just straight up fake. Sov citizens buy them, and cops often don't even pull then over because they're so fucking annoying to deal with.
Just a guess, but it's probably just because the person would end up throwing a tantrum. "YOU CAN'T ARREST ME! I'M BEING ARRESTED! I DIDN'T DO ANYTHING WRONG! WHERE'S YOUR PROOF!" (Or something like that.) As anyone around them tries to ignore the guy shouting at the top of his lungs and trying to do his best not to get handcuffed.