It doesn't give them the right to do that no, but doing it is still not necessarily a crime. The definition of "forge" definitely applies to someone signing your name to a check without your permission but the crime of forgery requires the intent to defraud or harm another. That other person could be you, the person that is going to endorse the check, or a 3rd party.
While generally there wouldn't be many times one business partner would need to forge their other partners signature, since they should have access to the money themselves directly, I can conceive of certain circumstances where it would be easier to just forge your signature with permission rather than track you down to sign something, and the "forgery" doesn't harm or defraud the person that is receiving the check. Spouses do it all the time.