It's part of what makes it the worst job to keybox. Unlike any other job that can use magic and can acquire and use all their spells at that level, BLU can only learn spells depending on their BLU skill. So, where, say a BLM, can acquire all their spells at 90 and use them to skill up on, BLU would only be able to use the spells they had the BLU skill to learn and then acquire more spells after they had skilled up. It's actually a great balance against simply keyboxing the job. And, why there are still so many shouts from Abyssea parties trying to find a BLU.
Personally, I think it would be great if all mage jobs were eligible to only learn their spells at skill level. It would end a lot of the lazy, gimp players posing as a FULLY leveled mage. Most responsible players would have no problem with it, since they skill their magic skills anyway.