It is not. Leaving your door unlocked is not inviting everybody in to take your stuff. You might make it easier for them to break in, but it still is a break in.

Making something available to the public is not the same as going to the google webmaster tools and telling them to index your page.

thats like saying "having a public website is an invitation to DOS-attacks"

there are conventions and reasonable expectations, until now I did not expect that a tracking-pixel would be the basis for crawling, so far most crawlers tend to crawl whats publicly linked, not whats potentially publicly reachable if one knows every url there is