Checked the forestry commission website, which states that it's a govt department. So it belongs to the taxpayer (us), along with the forests and trails.
It also states that the foresty commission "cares for" the forests, which does not equals "owns". So, I would suggest that even though there are people willing to pay to use trails, it would be very difficult to introduce and impossible to police. Are walkers going to asked to contribute to the maintenance also, since they're also permitted on all MTB trails in Wales?