It is pretty normal to compensate people for their time in my limited experience (both as subject and from what I hear about user testing). Usually it's a pretty small amount, not like a normal salary but a bit more than a cup of coffee for maybe 30-45 minutes of their time (assuming they don't have to travel to you in addition). They are helping you create a commercial product, it is not weird to do something in return, although I understand also what you're saying about how they might benefit from it themselves

Perhaps you could do a small amount, say (the equivalent cost of) a good cup of coffee, plus a free copy of the course so they can use it themselves and show some others? That spreads the word in addition to them feeling compensated

I'm not a marketing expert though, just going off of what I hear and experienced from other companies