my guess would be, that the window is used to determine the filling level inside the toner by shining light / laser through it. he says there are windows on both sides.
if the cartridge is emtpy light goes through, if not it's stopped. so closing the windows would work.
replacing the cartridge could trigger a check wether it's full or not. if the printer thinks it is a full cartridge, the page count is reset and it'll print again.
It'll be interesting to see what happens in another 1500 pages - what exactly will the printer compare to, and will you be able to reset?
I've had a three models of Brother laser printers, and each one did this. On each one I covered the window with tape and on each one I printed for a long time after. It is, the only real flaw I've found in getting a Brother laser printer over some of the more expensive options.
I'm not sure if Brother is being deliberately evil - I think it's more likely a matter of not having a cheap way to measure toner remaining and knowing a given cartridge is good for 1500 "average" page prints. The profit margins on new cartridges probably helped encourage them to not spend a lot of R&D on a more robust toner check method.
It's probably like you suggest - if the cartridge is inserted and is "full" of toner the printer assumes the cartridge is new and it resets the counter for the typical output.
If Brother had simply added an option on the printer to ignore the low toner warning, this would be a decent feature - warn me when I'm probably about to run out, but let me continue printing until I'm entirely empty. That way I'd have time to order a new cartridge but still probably be able to print for a while after getting the warning.
This particular "hack" is pretty well covered over the internet.
Exactly; I've done the same trick with Brother toner cartridges for quite awhile now by using black electrician's tape. The sensor is just an optical fill check, there is no communication going on between between the toner cartridge and printer (although the cost of the official cartridge could make you think it's capable).