I was always under the impression PT sells these as a way to market his testing services. There is a good chance users that care enough to filter will eventually have a need to get their peps tested.
It's hard to even turn a profit will multiple low-dollar orders and I don't think he has a minimum order quantity. If he is making 50 cents a filter and the average user orders 10 it is a lot of labor to handle the order, stock/reorder, deal with questions, pack and ship just to make $5.
I really view it as a service he is providing to keep us safe and healthy (just like his testing service). When I saw they were in stock last month I just grabbed 100.