What effect does taking radio towers have on weapon availability?

Right in the beginning of the game, Dennis tells you it's important to take radio towers because you can get guns for free in stores as a result. He's awfully unspecific as to how exactly this works, though.

Does each radio tower you enable, regardless of location, make free your bottom-most not-free tier of weapons, or do specific towers make specific weapons free, or what?

Furthermore, does taking over the towers have any effect on which weapons unlock (but still cost money) or which customization options are available for weapons?