Well considering it was fine when Clinton, Bush, Obama, and Trump were elected while the same ballot boxes were present it seems rather strange that no one brought it up or that it wasn't an issue until now. Surely someone in nearly 30 years would have said something, brought the case forward, or actually done something about the illegal boxes, that is, if such a law existed.
Not a fan of Biden by a long shot, but it does seem Republicans as of late are doing everything they can to make the election seem like it was false, rigged, or "illegal" especially when there has never been issues or problems before with the way people voted.
So either:
A) They knew that it was illegal, but treated it as if that law wasn't a big deal for 30+ years, not even warning their citizens not to use said drop boxes.
or
B) They are just now claiming that they are/were illegal after 30+ years for seemingly no reason other than to make some spectacle or point. That point again trying to make the election seem illegitimate in anyway possible, especially in a time of extreme political polarization in the US.
Even if they do find that the drop boxes were illegal for whichever reason, they wouldn't and shouldn't throw out votes just because people did something they weren't even aware was illegal especially when no one said anything in those 30+ officially or unofficially until now.