That is different... To the user, it appeared to be under wifi. I had this happen too, without knowing (warning) I was still connected to my wifi, but my network was down so all it did was pulled off of cellular.
Here, a simple warning requiring the user to decide if they want to download larger files over cellular would be adequate. Hell, they could even make a switch in settings under cellular data that "Allow large downloads" with it defaulted to off. If the user goes in, physically turns it on a popup could day "WARNING! DOWNLOADING LARGE FILES OVER CELLULAR MAY RESULT IN INCREASED CELLULAR DATA CHARGES".
And they'd still get sued. I'm serious. People would sue them or the EU would say they're being anti consumer or something and it's just easier to not allow it.