I don't know, was what I was told. Maybe a county or city rule.
Back when the valve was swapped on the 20 lb propane tanks I picked up a bunch from people's trash piles. You could swap them for a refilled tank with the new valve back then. Then they started charging a fee if you swapped in an old style. Anyone that wanted to do that I would give them an old style tank and a coupon that cost them $3 for the swap. I have 17 new style and 4 old style, only 5 filled.
I have a jet heater that I can hook up to a tank if need be. I used it a few years back when power was off for 2 days. It stinks and dries the air to much and had to keep some windows open. I had to snort water so I could blow my nose.