There is no point to QT if you're not going to follow a protocol of some kind. Many people buy frags that are only in fishless systems. Many people buy CUC that are raised in fishless systems. These systems are also fallow since they have no fish. If you just 'dump a bunch of frags and snails in your tank' the risk is not as high probably as 'dumping a bunch of fish' but it's still there. You are correct, though, there are many people that do not quarantine frags and snails, there are also many people that don't QT or treat their fish. Most of them get away with it - until they don't.
I find it hard to trust that these coral vendors with fishless systems isolate every new frag for 2.5 months before they put them into their system. I would doubt even more, considering 99% of CUC are wild caught, that CUC in a fishless tank don't carry pathogens.
But that's just me and my glass half empty attitude lol