Actually it is. Just like on st patricks day you can get pulled over or even breathalyzed walking down the stree just because its st pattys day. Same with 4th of July and other holidays. Besides if you tried to counter the "because its 420" argument they would just say your eyes were red or they smelled pot.I'm no lawyer but I don't think "because it's April 20th"
is probable cause for anything.