When people steal or destroy campaign signs from other people’s yards, it is trespassing. Destroying or stealing someone's property is against the law. It is also preventing their freedom of speech, which is in the Constitution. When signs are destroyed or removed, it also makes the opposing political party or candidate look bad, even those who wouldn't even think of doing something wrong.
When destroyed signs are left on the yard, it doesn't make undecideds want to vote for the other guy instead. And does tearing up a sign make people feel better?
As for those who have moved to oversized signs actually attacking a candidate (over the good-old American "vote for" types), here's a tip: It makes you look desperate, as if you don’t have confidence in your own candidate and have to be publicly negative. Again, this doesn’t exactly sway the negativity-weary undecideds.
The world sees America as a place where people live and let live, think positively and lend their neighbors a hand. Ripping up the other guy's yard signs? Come on, we're better than that.