I would expect this to be to allow market segmentation and price discrimination. A business might be willing and able to pay more for internet service than an individual, but less likely so if they could get the same thing for the individual's price.
I'm sure this is coming with Fiber. But for early stage products (technically Fiber only has three "customers" right now-KC, Austin and Provo), one of the best things you can do to encourage uptake is keep your pricing drop-dead simple.
I think they are just trying to balance that simplicity with keeping it fair by not allowing commercial servers. A couple businesses taking advantage of a system that wasn't built for them could wreck the speeds for everyone, and Fiber's reputation going forward.
Google is smart enough to write an equation that says what usage patterns are too much to be fair usage of their network for $70/month. Then they could (in theory) publish it as part of their Terms and Conditions rather than have a "non-commercial" clause. I wonder how that would work in practice...