I noticed this too, when I first reached the crafting instances. Yes, it breaks tradition with the previous three compendia, but as Tomiathon said, it is fairly easy to reach the required reputation level and unlock it. Still, I agree it would be better if this wasn't tied to reputation with some faction, as was the case the previous three times. Furthermore, only the prospector instance awards the compendum, which is unfair, since you might not have the prospector profession in your vocation.
In fact, if you dig this deeper, the crafting instances themselves are locked before reaching a certain reputation level. This means that if you have a crafting alt and want it to have access to those instances, you also need to rebuild at least a part of Hytbold.
The only consolation is that all this is fairly easy and fast to do, so it's not such a big bother. I guess we should be happy that we have crafting instances at all, given the low attention that has been given to crafting.
Violence is the last refuge of the incompetent.