Jokes are always confusing and could potentially lead to death. So how many people are we okay with dying so that we can tell jokes? Couple million seems reasonable.
(I think it requires a fair bit of mental gymnastics to come up with valid reasons to ban jokes on signs but permit advertising to drivers).
I still think it's a bit inconsistent to have zero tolerance for additional cognitive load from official signage while giving billboards a lot of latitude. Probably the most reasonable argument is that official signage should be held to the highest possible safety standard because in theory drivers have to pay attention to those messages.
But I don't think the government's actions are evidence-based. It's possible that non-serious, sports or holiday messages have a positive effect on driver safety, i.e, the negative effects of distraction are offset by increased driver vigilance when important safety messages are displayed. I don't think it's been properly studied.