Why not use owner's home network?
Even if so, uploading gigabytes of data every day is going to be noticed. While I'm sure they have the legal right to do so (via their TOS / EULA) they probably still don't want to emphasize that they're gathering that much data (unless you opt in to their fleet learning stuff).
Generally I think they do to access the Tesla App features. Tesla also have a network connection so they may already be uploading the information anyway over cell networks.
If not, I find it shortsighted and rather cheap that each car doesn't have it's own $50/month data plan (or whatever) to phone home important data.
To have a constant high speed IoT-like connection to the web for real-time updating and such?
I am sure some sort of volume discounts could be negotiated as well.