It's not a statistic (I have no sources to back it up), it's a sensible correlation that tech people will like tech cars. And tech people tend to have large data plans on their phones.
$5/Gb (per month) means, $5 per Gigabyte, and if it is not entirely used up in your one month billing cycle, it expires.
Why would I want a large extra expense for data I can only access while in the vehicle? I guess, if I was ride-sharing it would be a lovely perquisite for customers. But a small expense spread across a large number of people (lets call it the Netflix pricing model) makes a lot more sense. Especially since you have the system built anyway (AT&T, BELL, etc. etc.) I'm assuming this extra money (Dataplans in vehicles) is simply gravy over their business, so why gouge OnStar (where Onstar passes the gouging on to their customers, who then opt out of the service).
But what do I know, I'm in Canada, and we're gouged far harder than the USA in terms of mobile data.