For sure there are many riders who don't know the builders, and don't know that they should generally leave the trail as is, so can't be blamed for whatever "improvement" they might do to a trail.
Not everyone knows about nsmb, trailforks, even the nsmba.
a basic search on google using terms like mountain biking and north vancouver will yield the nsmba, trail forks, north shore ride and others on the first page.
in less than 10 seconds you can come up with the info you need to contact the right people to learn how get involved.
it's a safe bet that the people that cut the tree out had good intentions. i encourage people to get involved and can't really fault them for having that enthusiasm. however, i also encourage people to take steps to make sure they're doing the right things, including contacting the local trail association to find out how they can get involved.
apparently there's two trains of thought on this topic. one is that before going out and doing anything more than minor trail work like clearing mud holes or pushing dead fall off the trail, you make the effort to find out who's in charge before altering the trails. the second line of thought is that nobody has any expectation or right to be asked if it's okay for someone to do work on a trail they built or are currently working on and that it's essentially a free for all - you can build and change whatever you want.
now within the DNV, you are required to have a trail permit to do any work on a trail. however, as there are many land managers and some undefined/irregular boundaries on the north shore, it can be difficult at times to say with legal certainty that you need to contact the builders or other authorities before doing any work on a specific trail.
putting all the legalities and technicalities aside though, it seems to me that the decent and respectful thing to do would be to ask around before going ahead and doing any work. because this is basically an ethics issue in some circumstances (as the trail in question), there is no right or wrong answer. the only thing to guide us here is whether you believe a trail builder is worthy of the respect of some sort of contact before doing work on a trail they've created/ maintain or not. i firmly believe trail builders are worthy of that respect.
the only question people need to ask themselves is whether they feel that trail builders are worthy of the respect of some sort of contact or not.
I'm not a human in real life, I just play one on the internet.