As far as I know, you were right the first time when you said Creative wanted to keep the upper hand by being the only company to offer EAX 3-5.0. Of course, since I can't really contact the company and ask them about it, I can only assume this is what's going on.
When you say 'other manufacturers', you're talking about Auzentech - the only company to my knowledge that has been able to use Creative's chips in their third-party sound cards. Of course, if Creative were to give them everything up to EAX 5.0, they would be basically killing their sales completely if Auzen were to make a better, more stable product (which I feel they did).
So there you have it, to the extent of my knowlege. Like I said, I can't say that for sure, but even if they were asking too much for licensing, it would have been done purposely so that the other companies would either not be able to afford it or would have to give them a ridiculous amount of money in order to use EAX 5.0. Creative would win either way.