Probably because they have to test the ROMs on their network in different regions, with different frequency bands, with roaming, etc. That work is tedious given all of the phones they support.
Do they do all this testing for all GSM phones in the world? All new and old? All GSM phones ever sold?
I guess not. I guess instead they rely on the GSM specification to allow seamless independence between the phone and the carrier for 99.999% of the phones out there (if not more).
So why do they need to "test" the remaining 0.001% when they have a update in user-facing functionality the carrier will never see or interoperate against?
No carrier in Europe does this.
Does your ISP control what OS updates you can download? No. And why should they?
That carriers needs to do testing is a lie perpetuated to allow for customer-hostile business-practices. Stop repeating it.
It happens with Apple OS updates too, Apple just have sufficient market clout to tell networks they must complete the testing in the week between the gold master release and the public release.
GSM isn't some magic specification. It's entirely possible for a crappy radio firmware to cause significant disruption to a network it connects to (indeed, I've seen a pre-release firmware from a mid-tier Android manufacturer that managed to cause a reboot in every cell tower it connected to from one of the UK's networks). That's an extreme example, but carriers frequently do testing on that basis, and it often holds up the European releases of Android software updates.
The iPhone doesn't suffer from this problem. It should be up to the phone manufacturer (HTC, Samsung, etc.) to test their product.
Moreover, in some European countries it is normal to buy a phone and a subscription separately. In some countries bundling is even illegal. I have never had a carrier-branded phone since I switched to smartphones, including my Android excursion (Nexus 4, Moto X 2013, Moto G, Moto X 2014) and some Windows Phones that I played with (Lumia 710/920/640). They all worked on multiple carriers.
Everyone should stop perpetuating the carrier testing myth. For this we have the GSM standards and in many parts of the world a large number of phones are not carrier-branded.
