I work in a not particularly unrelated field and it's quite common to outsource the sending of emails and SMS, can help ensure they don't go into spam, throttle the sending etc. I'm a little surprised they'd need to use one for disruption messaging as they're usually much smaller volumes but sometimes it's easier from a system architecture point of view if BA doesn't have an internal service for it.

I'm surprised BA didn't fire them, don't think they'd have survived in my company unless they did something very unique !