Some cemeteries require a vault. The problem they'll face with no vault is the ground will depress there as things decay, and they want to get grass growing there ASAP and not have to worry about leveling things out all the time. But many cemeteries will allow it and will just heap the dirt a little higher so it settles more evenly eventually.

For now. Considering how niche the industry and how little the public has experience with it that seems like prime fodder for regulatory capture.

I bet with less than $500k spent on astroturfing and PR you could whip the useful idiots into a frenzy, get the requisite (bullshit) academic papers and get some key language inserted into some EPA rule that all but requires the vault in the name of solving some groundwater problem that isn't actually a problem.