Thats intended.I don't think this was intended way of it. Shouldn't these upgrades only affect spellcasters?
And now some history:
Spoiler :
Versions ago priest spells also requiered spell spheres and priests trained by govannon had an incredible amount of spells.
Then divine units got blocked.
After divine spell casting got tied to religion instead of spell spheres they got the ability to get trained back.
Then divine units got blocked.
After divine spell casting got tied to religion instead of spell spheres they got the ability to get trained back.