My post above was tongue-in-cheek. I have my doubts that data usage has anything to do with it. I think it might be more related to the likelihood of a corrupted download or something like that. I'm not sure why Microsoft would care how much data you use. That's between you and your carrier.
For people with large data plans or unlimited data, is there a way to bypass this and force everything to work cellular? Perhaps a setting toggle that can allow us to change back and forth?
No, there's no way to bypass the restriction as far as I know. If you have another phone, there is a workaround. Take your SIM card, put it in the other phone, and turn on the WiFi hotspot. Connect your phone to it, and download away. Of course this will only work if you have another phone available and you can do internet sharing with it.
On Android a warning that pops up when you try to do a big download on mobile data, warning you of high data usage. But you can confirm and keep going. Something like that, or a toggle, is what I'd like to see in WP.