I saw a couple times that a few posters mentioned they had never seen an equestrian out helping maintain trails. That certainly must be just where they live, as I am 15 minutes from a PA state park and for YEARS there was a strictly volunteer team of riders who went out every weekend to help maintain the trails for everyone. Once the state took over, they didn't do it as frequently, but the trails look fantastic for everyone.
I have never had an issue with bikers, and my horse doesn't have an issue with bikers, hikers, etc. The biggest "issue" I have come across is that I've had a few bikers ride too closely behind Nakai's rear, and he tends to scoot forward if that happens. When I've asked them to just let me know that they are there, they have often said they didn't want to say anything loudly for fear of spooking my horse. Better communication from both parties would easily solve that issue!
Also, bikers are to yield to both equestrians and hikers. Their bikes can't spook, and despite what has been said on their forum, even the most bomb-proof horse can spook if the right situation occurs. Also, wouldn't it be easier for a biker to move themselves out of the way rather than ask a 1,000 lb animal to move up or down (or around) and potentially damage more trail area, especially if it's on a slope?
As for horse manure, it is unrealistic to ask riders to carry around massive bags with them, particularly while out in state parks where there are no trash cans in the interior of the parks, or to ask them to carry pitchforks with them. Riding with a crupper isn't ideal either when trail riding. Horse manure fertilizers and it also deteriorates fast.