Given that BWG hot tubs is HQed in California, they might want to brush up on their state laws.
@rslade is envisioning the "Philips Hue" use case, which appeals to those who find a light switch burdensome. However, I see a much more practical use case. If my hot tub were to start diving towards freezing, I would want an alert so I could intervene before the repairs get expensive.
You miss the point - the Victorian's attempted to do this in the 18th Century, however, they had not invented the thermostat, which results in many scalding and deaths due to this fact. A hot bath, was a really cool thing then, but until the thermostat was invented, no one had even considered a need for such a safety solution.
If you read Bruce Schneider's latest IoT book, there is far more mobile and internet linked communications going on between partners, which apparently means humans are more connected than most would think. But I won't go into details.
The issues, we can do it, we have the means, we have the technology, why not? Who cares about the implications - privacy specialists and ourselves?