That is a new one on me about water heater warranties requiring expansion tanks in all cases or the warranty is void - makes no technical sense, but when you get the lawyers into the picture ...
The reason for expansion tanks, other than being required by code in some areas, is that IF you have a backflow preventer or pressure regulator on the incoming water pipe between the public water pipes and the water heater, when the heating water expands there is no place for it to expand to, and since water is essentially incompressible it pressures up your hot water pipes and can cause leaks or rupture of the hot water tank. I have seen several cases where pipes or water heaters split due to the pressure where the backflow preventer was close to the heater and the hot water pipe runs were short, so the expansion tank IS necessary if you have backflow preventing devices on the piping.
IF you do NOT have a backflow preventer or pressure regulator, then as the heating water expands, the hot water in the tank can push back into the incoming cold water line to accomodate the expansion.
Because more and more codes are requiring backflow preventers on house systems (usually either just inside the house or at the meter), it is becoming more common to see them required on hot water heaters as well as boilers.