One reason could be that if the course doesn't get the disc golfer revenue (no matter the price), they're not getting any additional revenue above and beyond their regular golfer clientele. In other words, they added disc golf as an attraction to increase revenue. The lower price point might be what they had to settle for to bring disc golfers out at all. $30 a round probably means zero disc golfers where as $5-10 brings them out enough to justify the expense of having the course installed.
I think it also depends on the facilities. If the disc golf course is essentially the ball golf course with baskets tucked off to the side of the regular greens, I see no reason to charge or treat the disc players any differently than the ball golfers. They're sharing the same fairways. They can't really co-exist in the sense that one group has to follow another regardless of if they're throwing a disc or hitting a ball.
On the other hand, if the disc golf course uses areas that aren't a part of the ball golf course (wooded areas between holes, etc) and there's minimal conflict between the two parties, there's no real reason to charge the same to disc golfers as ball golfers. On a busy day, they could probably put more disc golfers through the course than they can ball golfers...another reason they wouldn't need to charge the same to generate similar revenue. 100 disc players @ $10 a head = 50 ball golfers at $20 a head, or something like that.