My stance is that since they did the work, the authors of the library can license it however they'd like. But, if they wanted to get more people using their library, I think that they should rethink their approach. LGPL is more appropriate for a library, where you can still have your copyleft approach for the code you wrote, while still promoting wider use.
Here's an extreme edge case... as they said, if they get tired of supporting the email to get a free-as-in-beer license, they will just open it up with an MIT/BSD style license and be done with it. That's great. But what if someone gets hit by a bus? Or someone leaves the project and moves to Antarctica? There would be no practical way to release an unencumbered version.
Really though, they can do what they want - it's their code. But licensing is one of those areas that you really shouldn't try to be clever.
They actually don’t want as many people as possible using their library. That is not the goal when choosing the GPL. The goal is to maximize the number of free users in the world – that is, users who have the freedoms which define Free Software. Mere users is inconsequential. If users is what you desire, then by all means, choose a permissive license (MIT/BSD/etc).