I guess that depends on how you got your phone. Manufacturers make carrier-specific hardware a lot of the time since carriers buy in bulk. This is slowly going away in the US with most companies switching to pre-paid and BYOD plans, but still an issue. Sometimes it's simply a digit on the end of a model number, other times complete radio frequencies different. I remember when I got my Note2 years ago, I got the T-mobile one because it had the best hardware and most frequencies supported. I waited the required number of months before I could unlock it and hop to ATT for their network. Sometimes only certain model numbers will accept a flash because of whatever the carrier had the manufacturer do to their model.

I know there's recent legislation on it, but for a long time there were carrier locks on phones sold with plans and they were the only one you could have on it. Before LTE, CDMA phones couldn't switch carriers but with the grace of heir current carrier as it was on the carriers end. There was no removable SIM. I'm not sure if modern CDMA phones still require explicit requests to the carrier regardless of the LTE SIM as I'm on GSM which has always had them.

Modern android even has a SQL database file in their phones for things like carrier-locked-down tethering and limits. So if you have a version of android that doesn't have these bit-flags getting sent with packets or has new ones, I'm not sure your carrier would allow it on the network. Of course you can root it and make all data, even tethered, appear like it's coming from the android browser. Carriers seem to have a lot of hooks in the OS is all I'm saying, blindly flashing is unfortunately asking for trouble.

Most of the readership on HR doesn't. But most consumers want their phones to "just work" - unless they personally feel the pain of unpatched vulnerabilities, they expect that whatever is running on their phone continues to work. So it seems that the carriers have decided not to risk keeping their customers up to date (not wanting to take on any extra tech support). Carriers don't generally care what you're running on your phone, but since many handsets have locked bootloaders (and/or few people maintaining alternative ROMs), choosing a Nexus device means being able to do what you want, which could include being more secure.

> So it seems that the carriers have decided not to risk keeping their customers up to date

But I don't understand how the carrier is involved at all. What if you update your phone on WiFi so you aren't using the carrier at all. They can't intercept that so how are they to blame for whether you can upgrade or not?

If you purchased your phone through a carrier it comes configured to only allow upgrades approved by that carrier. At least in the US most people still purchase their phones through their carrier.