They can do it if they want. As long as
1) It's worth posting
2) it's not a scam
If a certain topic gains popularity, there should be a section dedicated to it. Within General, a Giveaway section should be made.
If you really think about it, if a section is dedicated to it, there's more specifics rules, and if it gains large enough activity, it's all just threads to giveaways and any member can enter rather than coming to General and looking for all the giveaway threads.
Tired of giveaway spam in general by people with 20 - 100 posts that joined yesterday.