Why Do Carriers Have to Approve Android OS Updates?

Why do Carriers Have to Approve Android Updates?

Prompted by Samsung's nano cement that the latest ICS update for the Note is ready... pending carrier approval, in the next 3months.

What is there to approve? If I connect a USB dongle, or have a notebook that takes a SIM card I'm still connect to their network, but I can do that via whatever OS I've installed, or updated to, but because it's on a 'smartphone' I need to get approval?

What am I missing?

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