Pardon my ignorance, but what are SEC rules for things like this? Do they expect some reasonable availability for trading platforms or do they consider massive outages like this "negligence"?

My limited understanding is that if you have a license to be a broker in this nature, you have to meet certain requirements. Similar to getting a bank charter - banks have to be open all standard business days during certain hours. Things may happen from time to time that are unexpected - like a massive snow storm that closes the bank for the day or at least part of the day.

I imagine the rules are similar - you must be able to have your clients trade during normal market hours. The regulators understand there can be unexpected events that put systems down unexpectedly (high volume, bug that crashes the system, etc.) and are generally pretty understanding. But I think something like this where they are down the whole day (and STILL down from the looks of their status page, though they claim to have identified the issue), could easily earn them a fine from the regulators for not meeting the license/charter requirements.