I think they think that since you're willing to give a tutorial, you probably aren't protective. The tutorial probably makes the problem worse. The old 'give them an inch, they take a mile' situation.

The suggestion below was good, though, the one that was along the lines of 'I don't mind explaining the general idea of how masks are made and I've made a tutorial to encourage those interested in doing so, but if you used my handmade patterns or tried to create a duplicate of my masks, that would be copyright infringement and I don't encourage that. I would like to encourage people to create their own art, not copy mine.'

The one below was much briefer, which makes it better, but I'm so lazy that I don't want to go find it and copy and paste it...