Or, more to the point, the user could look at the LGPL library, then write another library that meets the API and link to that.
The way that the LGPL is looser than the GPL is pretty much that the LGPL allows the creation of proprietary derivative works. It doesn't allow the distribution of them in a still proprietary form, but you can create them.
Linking does, of course, create the proprietary derived works that I mentioned.
Cheers,
Ben