I'm guessing it's more due to the damage that shredding can inflict on a trail especially when it's muddy, it can very quickly destroy features and carve large ruts in the straighter sections basically rendering them useless for when they finally dry out. I use it as an excuse to go explore abandoned trails and hunt out new shit when my normal trails are closed due to conditions.

 I can tell which one of you is a trail builder and which is a freeloader. Leaving ruts can change lines, cut off drainage systems and render them useless as well as rut out lips and berms and change their pitch, and the builders are left to clean up your mess. If this is ok with you, stay in the bike park and don't go anywhere near trails built by riders, you obviously don't understand how much work it takes to keep a trail running well.

 To me it's the way the local association is spreading the word. It encourages people too only ride in perfect conditions, which IMO goes against the whole idea of mountain biking. However, I am not saying that it is okay to ruin trails that people put hard effort into building. If they simply were to send out a memo saying what you outlined above that would be much better.

 I read it as "if you bike looks like this after a ride, you shouldn't have been riding". riding while its drizzling is ok, the ground just gets a bit tacky, no harm in that, but riding in a slosh pit is not cool.

Most people would not have the attention span to read what I wrote if it was sent in an email or on Facebook, lets face it

  Fair enough. I guess it depends how you read the picture, but I just don't want to misunderstood. I don't ever want to hurt trail builders work. Without them we wouldn't have anywhere to ride.

 haha! They close the trail if you sweat on it