So on an Android or Windows phone you can tether all you want even though you don't have a tethering plan?
Yes, because the carrier has no method in the phone's software to
prevent you doing it.
Windows Phone does have the same sort of tethering restriction, but Android doesn't.
If the carrier wants to stop you tethering with Android, they need to enforce the contract terms - charge you more, slow your connection speed, block anything they think is tethering etc.
iOS will check whether or not the feature is allowed on your account when you try and use it. Not all carriers actually do that (because it requires additional work at their end). This is very different to what carriers do on other devices - they don't have that sort of ongoing control over the device. You might find a carrier branded handset simply has the Tethering feature disabled, but AT&T has the power to disable it on any iPhone in the world with their SIM inserted (even if it's unlocked).
I'm trying to follow you here. Those are all services (minus FaceTime) that are provided by the carrier and not apple. Of course they will have control over it.
There's a distinction between you being contractually unable to do something and not being able to do it technically.
The important distinction here is that the iPhone was designed around AT&T's business needs - it was designed so that AT&T could control certain features on the device, while keeping Apple happy by not having any branding or carrier bloatware.
The by-product of this is that the system Apple created is a little "too good" from a consumer perspective.