How do spam bots spam exactly on questions that are relevant to their spam-ads?

.: How do spam bots spam exactly on questions that are relevant to their spam-ads?
Whenever i see a spam bot, i notice that they only actually spam on relevant questions.

Answers and Views:

Answer by Meklar
The simplest approach is to restrict certain spam messages to certain sections of Yahoo Answers.

That said, there’s no reason the bots couldn’t also scan for keywords in the question title in order to identify the most relevant questions. It wouldn’t be a resource-intensive operation.