"In the US at least, you do not need to register a trademark to enforce it... and one of the specific things trademark law lets you do is prevent people from misrepresenting the origin of a product bearing that name."
Unless you register, you cannot sue for damages and you are limited to state courts.
So how do you prevent people from using your mark?
You apply for an injunction. You would have to do this in every state where you want to enforce the mark. If you target multiple states, you would likely need multiple lawyers. How many of those free software authors will want to pay for this legal expense?
Until you succeed in getting your injunctions, and there is no guarantee you will succeed, there is no incentive for someone to stop using an unregistered mark. Whatever damage they cause cannot be recovered under a claim based on trademark law.
Unless you register, you cannot sue for damages and you are limited to state courts.
So how do you prevent people from using your mark?
You apply for an injunction. You would have to do this in every state where you want to enforce the mark. If you target multiple states, you would likely need multiple lawyers. How many of those free software authors will want to pay for this legal expense?
Until you succeed in getting your injunctions, and there is no guarantee you will succeed, there is no incentive for someone to stop using an unregistered mark. Whatever damage they cause cannot be recovered under a claim based on trademark law.