And even if the reason is obvious, I think it should still be given.
Well, like I said I see no benefit in bumping spam threads. The users should rely on the competence of the GMs/Mods in evaluating what is purely spam. Even better would be for users to never post spam, but I guess we have Monty Python and Tim Berners-Lee to thank for that not happening.
Another related suggestion, is perhaps GMs could be less strict on offtopic conversations.
I think we are too lenient. I know I am too lenient. OT discussions can be fun and they can even be constructive, but in both cases there is a possibility to continue the discussion under a new topic (that can be linked to). If the discussion is first on topic, then goes off topic, and returns on topic, many people who would like the on-topic information will miss the second part of the relevant discussion as they see the discussion take a new direction. People just don't have time to go through all the chatter when they are looking for something specific. Improved search tools would help (and I don't mean just in BB), but we should do our best to help with our own actions.
I feel the policy should change because it kills forum activity.
That's really up to the users IMO. Off-topic discussion serves mostly forum actives, not the masses.
there are often some great offtopic conversations relating to BB that happen because of an original topic sidestepping in to this new topic.
There is very much flexibility in such cases IMO.