That's the point. The idea of having a "defensive patent" is to avoid having trolls patenting the same thing and then going after small companies with no means to defend themselves in a patent suit. For instance: https://www.x-plane.com/2015/12/patent-troll-update/.
Anyone would be free to use the patent. But trolls can't sue with it.
The way the system is supposed to work is that simply publishing should be sufficient to prevent anyone else from registering future patents on anything that you've published. After all, publishing creates prior art which should invalidate any future patents covering your technology.
Anyone would be free to use the patent. But trolls can't sue with it.