Well, I can think of at least a few possible reasons. Not claiming those are right at all, but I do think they are the reasons why some people think/thought that scraping could be illegal.
1. It's hard to distinguish scrapers from DoS attackers, which we do expect to be illegal.
2. Depending on the use of the data after scraping, and the nature of the data being scraped, you may expect copyright laws and things like the GDPR to prevent any possible use of the data, so the very collection of it may be suspect.
3. Even for public data, a site may have Terms of Service that don't permit scraping, and some may expect to be able to enforce these
1. It's hard to distinguish scrapers from DoS attackers, which we do expect to be illegal.
2. Depending on the use of the data after scraping, and the nature of the data being scraped, you may expect copyright laws and things like the GDPR to prevent any possible use of the data, so the very collection of it may be suspect.
3. Even for public data, a site may have Terms of Service that don't permit scraping, and some may expect to be able to enforce these