I don't see why this is considered bad?
A search engine shows the user content that is publicly available. If that content is not publicly available, but you make it appear to the search engine as if it was, then you cheat. You don't think that is a questionable way of doin things?
Besides, you only annoy people with that. Maybe they will make an account on your site to see the content. Maybe they will even use a trashmail for that (and don't bother, you will never manage to ban all trash mail providers), then they will see what they want and will most likely never come back. Congrats, you have a new dead account in your system. The other thing is that someone who is annoyed is nt likely to come back to your site. He will use the search engine again when he has a similar problem and then most likely visit another site. But if he was pleased with what he found at your site, he might considered searching for a solution to a related problem directly on your site.
And as i said, is the goal of a community not to have _active_ members, members that feel comfortable on your plattform and contribute to it, instead of just bloating the members list with dead accounts?
And last but not least:
You can fake browser strings yourself. Cheating at your board and making it believe your a bot is not that hard at all. Because technically a bot is just someone who browses your board without logging in. I can do the exact same thing and your board believes I#m a bot. And i see all your nice content that you wanted to hide.
If you take all that into account, i don't think such a function will be implemented. Besides, you _could_ possibly write a plugin for that. You can check wether a user is a spider with an event listener and generate a session for it if you want to. So way every bot could be logged in and have rights from a user. you could place this user in a separate user group and be just fine.