I think this (as usual) is more complicated than it seems. I have an iPad without a data plan that is set to automatically tether to my iPhone. I have an AT&T Unlimited data plan on my iPhone, but that plan has a limit of 30GB of data for tethered devices.
We wouldn’t ever have a way to quantify this but even if this was 1% it wouldn’t be fair to change the default to create a situation where they’re accidentally paying an extra $100 or $1,000 bill because they had accidentally downloaded some shows on cellular without realizing it.
We plan on making downloading over cellular an option in the future, it’s just going to be something that we have to be careful about to prevent it from hurting folks who don’t have these unlimited data plans.