Some people, mostly plumbers and drillers but homeowners too, don't install a prefilter. I've torn them out and replaced them with a backwashed version.
OK, So "some plumbers, drillers and homeowners" don't install a pre-filter. That just about covers most of the folks installing so who exactly does install a pre-filter. I wish you would stop making blanket statements designed to malign whole groups of people. BTW, I have always installed a pre-filter with and upflow AN filter because the instructions are pretty clear in telling you that you need to so a better statement on your part would be "people that don't read directions don't install a pre-filter" Oh and internet dealers don't install anything.
Hey man, there ya go gettin' all girlie again. If you don't like how I write, don't read my posts, we'll both be much happier but that vision of you, a MASTER plumber since ya was a boy in daddy's shop 3-4 decades ago reading installation plumbing instructions, now THAT'S FUNNY!!
BTW, you quote about as good as you troubleshoot. Probably because troubleshooting instructions can't cover all possibilities so they only list the most basic things to look for and the rest is over your capability. LOL there's that visual of you standing there leaning left reading troubleshooting instructions!! Ya know, you can worry about that instead of who did/does or didn't/doesn't do whatever.