It's worth adding that big (BIG!) business clients will usually negotiate the terms for going below the SLA threshold. The goal is less to be compensated if it happens, and more to incentivize the provider to never let it happen.

Right. Basically, they give you a coupon to lower your cost of future consumption. So, you have to keep consuming the service. If you just leave, you get no rebate. Obviously, very large customers get special deals.