They fall under the Grey Areas paragraph of the Concepts section in the Wiki Rules.
I agree that the wiki could use some more page types, right now concepts are pretty much a catch-all for anything that doesn't fit in the other sections. Articles on subjects like turtling and grinding are totally valid in an encyclopedia of gaming knowledge and we don't currently have a better place for them, which is why they have been allowed as concept pages ever since the site first went live.
I'm going to close this topic, it can be addressed again if the situation changes.
Well grinding is a concept related to video games so it should definitely stay in one fashion or another.
By that logic, we should also have a page for speedrunning. (Then again, I can imagine that working under very specific conditions. Braid comes to mind.)
We already do, it was one of the first concepts ever added to the site and it's really good too (but it's one of those pages Mento talked about above that can't have every game linked to it, they need a good reason to be attached): http://www.giantbomb.com/speedrun/3015-179/
We also have concept pages for camping, turtling, grenade spamming and numerous other player behaviours.