For example there is IPB with closed access to access board you need to register and have admin to activate your account.

Although when you do all this steps and get access to the board and post there, you can after few day's find your post trough google search engine because board does not restrict web spiders and probably other users access to printable (no graphics) version of board.

Is this bug?

+1 I want that fixed if it is

The admin decides how bots can view the board, and what group they are treated as.

Ok cool so this is no bug

The admin decides how bots can view the board, and what group they are treated as.

Yeah, but all this rules are disengaged when board is viewed as printable version.
At least i have not seen any way to stop this behaviour.

Use a robots.txt file.

User-agent: *
Disallow: /index.php?act=print

You can only view the printable version of a topic if you can view the actual topic.

If spiders can see your information (and you don't want them to) it's a configuration issue. Submit a ticket for some assistance setting it up so that spiders are unable to read the data on your site.