I have tried that and doing a whois on my public IP revealed that it belongs to the original ISP, not whatever local ISP I was using. Even visiting Google shows the Google page of the original country. I guess they just have agreements to tunnel traffic in a way that circumvents the Great Firewall.

But politically it is much easier to understand. The only people who are likely doing this are tourists, and the government certainly doesn't want to hurt tourism.

>a whois on my public IP revealed that it belongs to the original ISP, not whatever local ISP I was using

did you do other checks to see if your traffic was being censored? a whois of your public ip says nothing about whether your traffic has been manipulated.

> Even visiting Google shows the Google page of the original country. I guess they just have agreements to tunnel traffic in a way that circumvents the Great Firewall.

google serves pages depending on your public ip

The Great Firewall doesn't in general modify traffic. It either allows traffic to pass though, or blocks it (TCP RST, rate limiting, etc). I was able to visit normally blocked sites like Google at full speed of a typical cellular connection.

It works because cellular roaming traffic travels over a tunnel to your carrier's source country. This greatly simplifies data billing calculations, since they only have to keep track of the usage of one connection.

The traffic is already tunneled when it passes through the GFW.

So I guess the reverse is true also, which explains why my mainland SIM card wouldn't have unfiltered access roaming in HK.

I believe so. When I was in China, my mainland SIM didn't work at all in HK. I assume many people get prepaid SIMs when traveling out of China so they wouldn't be subject to filtering anyway.

Isn't this effectively a VPN? Or is the traffic not encrypted?

Yes, effectively a VPN but I would just call it a tunnel, I don't think it fits the definition of a VPN.

I'm not sure if this traffic is encrypted (I assume it is), but I know GFW filtering happens at egress points so mobile traffic would already be tunneled by then. They could add dedicated hardware at mobile towers to GFW this traffic but I don't think the govt sees a need to. The primary target of the GFW is to stop Chinese citizens from accessing foreign services. Foreigners on foreign roaming data plans are not the intent of the GFW.

My bet is that they could filter this, but it's not worth the infrastructure investment.

The other half of why this works (everyone else has already covered the technical end), is that the party is not really concerned with controlling foreigners' access (at least not nearly as much as they are concerned about domestic). So they focus their efforts at blocking traffic that isn't liable to be 99% non-resident.