Flow meter: Why not before backflow?

I’ve read that the flow meter should be installed after the backflow valve. In the five years living in my house, the only system leaks I’ve had (2x) were when impurities (gravel/plastic) from the water main line somehow got caught in the rubber washers in the backflow and resulted in a strong flow of water pouring out of it when all the zones were inactive. It seems like the flow meter would be useless to detect this, since the water was not flowing to the zones in this case.
Why can’t the flow meter be installed before the backflow to watch for these types of leaks?

I think the main reason is because it is not rated for use with potable water. So after the backflow would prevent water going through the flowmeter from being used within your house.