All of these services seem to take the same approach: AWS US-East, maybe use two availability zones, call it a day.
PagerDuty had to learn this lesson the hard way (circa 2012), as they were entirely in US-East during the catastrophic AWS outage in June. They proceeded to go offline along with a lot of their customers, who subsequently were not alerted.
Multiple region redundancy is not trivial, but it would certainly be a little higher up on my to-do list if it was basically the entire value proposition of my service. I was a little suspicious of PagerDuty's operational chops before, but now I simply don't trust them.