Most but not all controllers do this.
For example, the built-in BMS of my new LiFePO4 battery pack will allow at least 1/4A backflow at night. I've made sure that all strings connected to is have blocking diodes built in or I add them.
Not wishing to be argumentative, but BMS != Charge Controller.
Unless I've very misinformed, the BMS on the LiFePO4 packs are there to balance the voltage across individual cells. As such, your power is supplied pretty much straight across the series pack and the BMS just tries to "partly bypass" high-voltage cells so the others get a fair charge.
As such, discarge would be entirely possible, nay inevitable.
"Charge Controllers" like the FlexMax 80 I use are substantial devices (in electronic complexity) performing DC-DC conversion, MPPT, logging etc and most certainly DO prevent backfeed to the panels.