This is a poor reason to give positive trust. So does that mean that I can run contests and giveaways in order to build my trust here? No, it should not mean that. There is no risk participating in a giveaway or a post, and it is not a trustworthy action to give away money or to sponsor something...for this reason is why so much corruption exists in the world through political donations and lobbying. Is that the same kind of system that this community should follow?
I mean, you can if you want, no one can limit you.
There's risk of participating giveaway, as I mentioned before, not all sponsor will distribute the reward. If the sponsor delay in distribute the reward or not give the reward, they deserve to get negative trust right?
CMIIW, they didn't force the winners to left positive trust, so it was the users. If you can caught them asking for positive trust, it will be an another discussion.