No, this is not the reason. The reason is that the websites see that request originates from an IP address that is hosting a Tor exit node. The list of Tor exit nodes is public, because Tor clients need to know them.
What you’re probably getting at is “exit bridges”. See these posts:
- Using hidden services to replace exit nodes - #3 by gus
- Websites blocking Tor
- How to circumvent Stack Exchange blocking Tor users from registering accounts? - #7 by everydayisoks
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