This has been AT&T's policy for 'years' now.
I had an old HTC TyTNII which had the old 'pay per use' data plan - in which I was able to disable the data and use it as a WiFi smartphone (back when 3G was not very well built out or stable)
When iPhone and Droid phones started becoming highly sought after, AT&T basically stated that phones are either 'feature' (plain phones) or 'smart' phones, and all smartphones (they don't care where you got it from) are subject to a smartphone data plan to be used on a standard plan.
The only part I hated on that - was typically if I switched phones, even temporarily, I'd be subject to 'scanning' and having a different plan applied.
It now gets 'worse' - to the point that there are 'iPhone', 'iPhone 5', and just about every model/make plans.
Why ? Because you effectively 'could' keep your plan for a very long time, and AT&T would like to make sure that if you have a new phone, it gets a new(er) plan and rate
Verizon and Sprint never had to deal with this in the past, as all phones were driven by ESN, and 'unlocked' phones may exist, but carriers wouldn't activate them. GSM based AT&T/T-Mobile use SIM cards which allow you to swap phones. So instead, they're using the IMEI (phone identifier) to id the model/type and be obnoxious through that means.