There's no law against you (or anyone else) harvesting a few of those signs. Coroplast is useful for a number of things*, and political signs are a source that comes free.

* For example, the reason we didn't get drenched by this morning's rain is that the holes in our roof from Hurricane Ike -- which we have not yet managed to find a contractor to fix because they're all swamped -- are being covered by political spam signs. They work much better than blue tarps.