50GB spread over 150 million users comes out as ~333 bytes per user and day.
Of course, the transmission of that data is likely more bursty, but even if it transmits all the data in one go, that's only 10K per month.
So your argument about the limit doesn't really fly because even if they did charge for for that data (which they probably do), considering a limit of 1GB per month, those 10k would be 0.001% of your monthly allowance, so it's probably not even detectable by their overcharge detection algorithm.
Now. Don't get me wrong: This kind of malware is really bad and shouldn't be on these phones, or if it is, it should be opt-in for the purpose of remote support.
It's just important that we hate it for the right reasons (security, privacy).
The "150m devices" claim is rather vague too. It will no doubt include devices that are no longer in use, like when a dating site claims to have X million members without mentioning the fact that all but a few thousand of them haven't logged in for many months - they can truthfully claim such devices have the software installed but that will skew the average bytes/device/day taken from the released figures downwards.
A rule of marketing (which includes selling the company to prospective employees if they are looking for them on linkedin): Never lie when you can selectively use honest statistics instead.