Just my 2 cents on why you cannot reset the counters.
It is generally considered a poor practice to reset the counters due to the fact that multiple management stations and applications could be monitoring them. Each application is expecting the counter to be >= to its previous value.
So in the SNMP world, it is expected that application itself will do the math. Zeroing the counter in session simply tells the application to record the current value of the counter and subtract that value from all future values before the value is displayed.