As stated multiple times above: there is a law. Can't ban signs without other repercussions for free speech. Worked on a campaign and it comes down to three things:

1. signs are a waste of campaign $ generally but the "base" i.e., core supporters/volunteers love them so they print them.

2. any $ not spent on signs would end up in your mailbox as political mailers.

3. both parties equally suck on both 1 & 2.

Such is the way in politics in this country. Support 3rd party candidates and campaign finance reform.