There is a difference between random topics that encourage conversation, and random topics that only encourage spam. If the topics look like they are spam, and do not encourage any type of constructive feedback or valuable conversation, they will be locked. Random topics that do encourage longer posts and conversation however, will not be locked.
So, in conclusion: it is not always the randomness of the topic that gets it locked, but rather the nature in which it is presented and the sort of responses that it encourages.