SLAs can be meaningless like that. However the better ISPs have in place a backup system that doesn't use the same fiber/wires. Sure the backup might be a radio or satellite feed and so be slower, but it will get/keep you online. This costs are lot more per month though, so if you are not paying for that service your SLA will probably just be we give you a free month (which hurts them enough that they will do some things to prevent downtime, but not enough that they put redundant fiber paths in the ground)