Earlier quoted context omitted.
What use is SLA? If a service goes down for too long, are you really going to hire a lawyer sue it over SLA or just... use another backup?
It's not about suing, but defining expectations about how you can rely on a service. For example, my team has people across the world for HW bringup, so we can't allow our code hosting or CI to be down for more than a few hours. Of course, backups have different uptime requirements, but as for everything, it's a tradeoff between features, of which an SLA is one. Tarsnap's features are granularity of cost, reliability…
Meeeeh, my ISP cut of around 100+ fiber connections in my town and spend three weeks fixing it. My neighbor have business line, there's an SLA on those that among other things, require them if reestablish his connection within 3 - 5 hours. It took them over 500 hours, so that SLA is useless for anything but forcing compensations.
The problem is that the SLA should give an indication of available resources, but in reality it's mostly a contractual thing for most companies, they'll pay the "fine" or refund a customer if they fail to hit their SLA and that's about it. Tarsnap most likely have better availability than many midsize competitors simply because it's just one person who really cares about it. Doesn't help if he's hit by a bus though.