I thought the reed valves keep the combustion force from shooting back up the air/fuel intake during the combustion stroke.
Not exactly. The reed valves are not exposed to combustion at all. They sit between the intake manifold and the crankcase and are simply one way valves to let air into the crank case when the piston is on the way up and generating vacuum in the crankcase, and to prevent air from escaping when the piston is on the way down and pressurizing the crankcase. It's that crankcase pressure that forces air into the cylinder when the intake port opens. The pressurized air coming in displaces the combustion gases which have nowhere to go except out the exhaust port.
The whole system relies on the momentum of the air to work well too and this is where expansion chambers in the exhaust really help. Their size and shape give the exhaust gases a wonderful path of low resistance when the exhaust port first opens so the gases flow really quickly into them and then start to cool as the gases expand and exit out the exhaust pipe downstream of the expansion chamber. This creates momentum of the gases flowing out and a vacuum inside the expansion chamber that, due its size and shape, coincides perfectly with the next opening of the exhaust port which makes the exhaust gases flow really quickly etc.
It's at the RPM that the intake and exhaust ports, and expansion chambers are designed for that the engine makes best power. The downside of this is that if you aim to make all the parts work together perfectly, the engine can become very peaky, power-wise, so the ports and chambers are typically not all designed to work best at one RPM. By trading some peak HP at one narrow RPM range, the engine can be made to make a smoother power curve that's more useful in the real world. If you have ever used a chain saw, you know what a peaky two stroke feels like, it's either screaming, or bogging down like an on-off switch.
I think boat two strokes (and four strokes for that matter) create a lot of vacuum on the exhaust port by running the exhaust through the prop hub rather than through expansion chambers, which are bulky. At speed, this creates low pressure that draws the exhaust gases out. It's also not very RPM dependent.
There are a dozens of other factors that come into play with two stroke design too, I am just scratching the surface here. Gordon Jennings wrote a great book about two strokes, if you find this stuff interesting: See
http://www.amrca.com/tech/tuners.pdf
