You're saying that many of these people have no WiFi at home, and stream unlimited pirated HD videos from their phone to their TV's via AT&T wireless? Presumably buying an Apple TV just for Airplay, or do they use the lightning to HDMI adapter? It seems to me that those people would be a minority of users.
Yes, that's what I'm saying. And while they may be a minority of users, they are a majority when it comes to the amount of data they consume.
Last year I sold the SIM card from my iPad with the Grandfathered UDP for $1500 because the buyer had no wi-fi, consumed large amounts of data, and $30 a month was the best deal to be found. Check out eBay and the iPad section of this forum, prices were once even higher.
We have 7 iPhones and 4 iPads that can do native 720p or 1080p video, and we usually only sync SD video to our iPhones and put HD video to our iPads through iTunes. I can see why many people would prefer to stream HD to their iPads while they are mobile, but agree that's it's less important on a phone.
Similar here, we have 5 iPhones and that 1 iPad on our Unlimited Plus plan. To see if they'd notice, I put my 3 kid's iPhone's on Stream Saver and they've never been able to tell the difference, that what they're streaming on YouTube or Netflix is only 480i.
Big picture, Stream Saver is a blessing. 99% of the users couldn't tell the difference anyway, and imagine what the network would be if millions of ATT users took advantage of the new unlimited plans and could stream in 1080p or 4K. Would choke the very reliable network most of us are so pleased with.