They aren't, but there is a lot of misinformation (rarely, if ever, intentional) out there.
I haven't much time this morning, but a condensed version:
-A back horn (viz. loaded by the rear of the driver) is typically a bass horn. They aren't much use for anything else due to group delay considerations and as a rule of thumb, have an upper operating limit of about 300Hz, above which you need to be rolling them off to avoid issues with the aforementioned group delay. The exact frequency is not set in stone; this is partly subjective, partly system related and the amount of delay is inherently a function of the horn path length.
-A chambered back loaded horn is in essence best viewed as an extreme variation of a vented box or bass reflex. Ergo, if a driver is suitable for a vented box, it is also suited to a back horn. Contrary to widespread myth.
-The myth of low Q drivers being required comes from a couple of places. Firstly, front horns (viz. loaded by the front of the driver), where it's often / usually beneficial. And secondly, many early moving coil horn drivers had very large motors and a lot of electromechanical damping. So there was an assumption that was what was necessary. Unfortunately what people missed was the other half of that particular matter. Those drivers were designed with large motors partly because power was expensive, and partly because the amplifiers they were designed to be used with had a high output impedance, which has the effect of artificially raising the driver Q. Effective Qts = Qts + any series R (or amplifier output impedance). Many amplifiers actually had an adjustable output impedance to allow easy tuning.