I made the same complaint in a previous "someone just rediscovered Ada" quarterly thread, and an Adacore dev replied that most of their lib are now GPL + linking exception (well, he didn't say "now", but on their former Web site it's pretty clear no exception was mentioned anywhere regarding their OSS code, so I assume there was a change recently).
I agree with your sentiment in that, Adacore being into high end consulting, I don't see what they'd be losing by relaxing their licencing. This would bring in new people (I mean, if there's a new "hey I just found out about that Ada thing" thread popping up every few months here or on Reddit, clearly the language has something going for itself), which is clearly something they want to see happen.
Ask GreenHills, PTC, OC Systems about similar contributions.
Unless you plan to write 100% GPL software it's pointless. The FSF version of GNAT under mGPL license is always lagging behind and many essential libraries are still under GPL. Moreover, you will be pretty much locked into AdaCore as the compiler producer even with the free version, since AdaCore is the single main contributor and pushes the latest standards like Ada 2012. AFAIK, no other compiler supports Ada 2012, and the other compilers are proprietary and expensive anyway.
AdaCore is at the same time the biggest maintainer and biggest destroyer of Ada. They make sure that only the GPL version is easy to install, easy to get, fully maintained, and every program you deploy that is compiled with that version is automatically under the GPL.
That's why Ada is slowly dying and I'm not sure the good folks at AdaCore really get it.