Bitcoin price goes up and down all the time. If someone buys something costing 10$, the next day it may cost less or more. So it's a risk for the seller.
Businesss accepting Bitcoin as a payment means, is a milestone we have been waiting for, and it would be a great thing if such happens.
Talking a about the price fluctuations, you've hit a point, but that's what we have cryptocurrency exchange platforms.
If we are looking at a global adoption of Bitcoin, there should be a means of integrating these businesses to exchanges, to allow for the instant change, to fiat, whenever a user makes payment with Bitcoin. This way, the seller doesn't have to worry about price fall, unless he decides to hold the Bitcoin for the long run.
I really think it would work.