A great many things are sold this way, and it is sometimes unavoidable. Commercial projects tend to have a wide range of requirements from a licensing point of view so if they spent time drawing up a full commercial license they would end up having to rearrange half or more of it for the first commercial client that came along anyway. They could spend time drawing up many license templates (world wide, X,000 users, X server, ...) and pricing plans with many combinations of support and development agreements to try cover the bases - or they could spend more time coding.

Their preference is that the library be used under the GPL. That is their choice and it is a fair enough choice. But they also acknowledge that some entities can't accept that for one reason or another, and are willing to be flexible. Very flexible: instead of saying "this is our commercial license, deal with it" they have said "tell us what you need, and we'll see what we can do".