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.