It is much, much easier to hide such in proprietary software.
That's a good example. Because it is open source, any such malfeature will come to light.Like Ubuntu does with its Amazon dash? Aye.
Then users are much better equipped to decide whether to use it.
I mean as an user, not as the web admin. You can't see vBulletin's source unless you've paid for it.Because there is no closed-source bulletin board system. Technically impossible.
(technically, you can have a closed-source forum - CGI still exists)