The ruling can be found in Martial Law.
From the rules written in ML everything is perfectly clear. You need to be able to cast the spell to inscribe a rune and thus only those with access to a given sphere will be able to create runes from this sphere. I am fully of aware of that ruling and have no questions concerning how it works.
Why Rasyr made a contradictory statemen on July 16, 2008 I can not say - but since he did not flag it as an official ruling I would say that the words in Martial Law hold.
Non-spell users should not be penalized by having to invest twice in the often costly Mystical Arts category in order to be able to inscribe a rune. So the requirement to be able to cast the spell was planned to be dropped (this got lost on the way to the final book, as Rasyr explained above). This is what Rasyr clarified. Now we can well understand this reasoning and used the skill accordingly, but then the question arises how to handle the restriction concerning which spells are allowed to be inscribed. And concerning this new ruling I'd like to know whether a restriction concerning the spell spheres does still exist.
Note: Your warrior could go to the high mage and have them inscribe the rune, and then the warrior can activate it when needed.
Yes, I am aware of that. And since the Mage would also be able to create a rune with a certain trigger condition, the Fighter would even be able to use the rune without having to make a Runes / Battle Runes skill check in order to activate the rune. But you don't always have a Mage/Bard/Ranger/Druid etc. at hand...