It all depends on who is responsible for the trails. Some places you either ride them muddy or don’t ride. Those, who frequent Snowshoe, know it’s a grease pit most of the summer. If Snowshoe closed every time muck or mud was present, they would be open fewer than 20 days a year. The hillside swamp, known as Diablo Freeride park, would experience problems staying open, too. The PacNW can be mucky. Whistler opens for biking in late April or May. They literally shovel snow off the trails. You think the trails are dry? In the early days of summer, on River Run trail towards the base, there is a ladder bridge that is supposed to armor the trail. The snowmelt can make a mud pit that covers the ladder. I have actually seen them remove the ladder (I’m guessing safety reasons). We were bombing the trails at Winter Park during a downpour this summer, covered head to toe in mud. The trail builders were cheering as we blasted by. Granted those are all maintained by private funds.
Here’s some public access notes. There’s a trial in Colorado high country with a posting that states, "if your tire sinks 1/2 inch in the mud do not ride." That is an incredibly, mucky trail. Two sections of trail in Golden Gate Canyon have been overrun by adjacent creeks. The only time those sections aren’t mud stew is when they are frozen. Conversely, you’re terrified of trail closures if you leave tracks due to some hikers spilt cup of coffee in Fairfax County, Va or Jefferson County, Co and you should be. Fairfax county is a swamp. Bikers riding wet conditions deepen puddles or widen the trails. These puddles stay for a longer time ruining the experience for hikers. Jefferson County has lots of clay. The arid nature of the high plains causes the clay to dry quickly and nearly as hard as cement. Tire tracks and hoof prints cause nasty ankle twists for trail runners. Trail over-use compounds problems in those counties.
Every region and many trails in each region has their own rules for wet riding. Generally, pay-to-play riding areas have more liberal wet riding standards. If Subaru took that picture on private property in Georgia, that waits for the first rain of hurricane season to hold the "Mud Bog 12," who are we to criticize them?
Wet weather riding does turn some "blue" rated trails into triple black diamonds. Individuals drawn to mountain biking to test their skills and not their stamina, will always have a short love affair with wet rather riding. Espousing the deontological ethic regarding the naughtiness of riding soupy trails bears similarity asking Senators for term-limitations or a balanced budget. Thus, this really is the best part, "And with help from our dedicated partners, you can be sure there will be plenty of muddy trails and a means to enjoy them for a long time to come." Trails targeted at riding wet and muddy should be supported by private funds. If more private funds were pushed to building trails for wet sloppy riding, you would see far less riding on trails poorly funded by over-stretched tax dollars.
If you’re curious I hate riding in muck. That’s part of the reason I moved away from Virginia. The heretic monkey will stop flinging feces.