Wow hadn't even considered this... so say I have a members only section of my site where I share high value content, one of the members browses using Comet, and that scrapes the private content and sends to perplexity?

Not sure if its still an issue, but companies were buying popular web extensions, then auto updating malware/spyware into them. I haven't heard much about this in a while, but I think chrome still forces auto updates for extensions, so I would expect this to be the biggest vector for scraping walled data now.

Any user could manually download your data anyways. Access is access.

This also happens with covert botnets running secretly on user machines.