That's a problem for site operators not users if more people force sites to switch to HTTP's more AdNetworks will start serving content via HTTPS.

The good thing is that at least most browsers these days are good at blocking mixed content so you won't have HTTP adds growing tumors due to injections anyhow.

What i don't understand is why AT&T has to do injection in the 1st place they can easily setup their own AdNetwork and partner with the large AdNetworks to route through AT&T's own internal servers.