Whilst true the normal case in the US is to be charged for briefings and debriefings and ground instruction as well as flight instruction.
Therefore witha two hour training slot you might pick up an instructor charge for two hours and an aircraft charge for one hour.
Actually when I was at OFT both the instructors and the planes were charged, separately, based on Hobbs time.
So if you had the plane and the instructor for a two-hour block, and you flew 1.1 hour in that block, then you would pay 1.1 times the hourly plane rental fee, and 1.1 times the hourly instruction fee. If you flew solo, you just paid the hourly plane rental fee.
Of course I don't know what the payment scheme to the instructor was, but I think this charging scheme is probably there because it neatly ties in with the website advertisements for the 45-hour PPL package (including 25-hour instruction).