This happened in Roanoke, which is left leaning. Republicans are going all out to cheat in this election.
Edit: Here is the law on tampering with ballots in Virginia. Class 5 felony
https://law.lis.virginia.gov/vacode/title24.2/chapter10/section24.2-1009/
The key point is that if it’s not signed, it gives them an excuse to throw it away.
Do that to enough people and you end up with a big problem. The uncounted votes could swing the election, or you get a messy court case where some judges step in and basically decide the election on their own, or who knows what else.
At a minimum, it’s sets the stage for chaos. And from the chaos can emerge a chaotic outcome.