The purpose of the policy is primarily censorship and not the creation of localized monopolies. For example, just several weeks ago several domestic VPN service providers were shut down by the government. The population of people choosing to use VPNs aren't significant enough to damage any market, the only purpose would be censorship. Also if market protection is the purpose of the policy it should have relaxed as domestic services like search engines, social media, and online merchants have established themselves, this is not the case at all, the great firewall has only been made stronger compared to a decade ago.
The development of different markets are also not that simple. For example, it's not like eBay or google didn't operate in the Chinese market at the same time as Taobao and Baidu. They were driven out by the domestic competition for various reasons (google suicided by deliberately uncensoring itself, but that was just a particularly flashy way for them to leave, the underlying reasons for that was still market driven).
If you expand to look at other markets, it's very clear sometimes market conditions are just different, and even huge transnational companies aren't somehow fated to "naturally" expand everywhere. For example, central HVAC systems is basically nonexistent in China (and from what I understand, other similarly developed East Asian countries like Japan and South Korea), heating and cooling is done by machines scaled for use in a single room and tuned on/off individually rather than from a central unit scaled for a building. On the other hand, technologies in the electronic payment industry are far more developed, I see QR code and NFC based payments used everyday in China, while I've pretty much never seen such a thing in the US outside of stuff like the DC Metro's SmarTrip card. I don't think I've ever personally seen someone use Apply Pay (back in 2016) and even smart card based credit/debit cards took until something like 2010 to arrive in the US.
There are certain practices like currency manipulation and Chinese steel dumping that are unquestionably meant to be unfair market practices, internet censorship just isn't one of them. Besides, Amazon or google doesn't exactly need any help in the US market from what I can tell, what Chinese companies/websites would you suggest blocking?