It's unfeasible or at the very least hard work to check everyone's posts individually especially when you have this many participants and this is just their way of trying to cut down on spammers. Most people who are contributing relevantly won't have much trouble running into the minimum character limit anyway. Also, making short posts doesn't necessarily mean you're a spammer but bitmixer have decided just to not pay for these posts. Though some people may think its harsh or unfair I think it'll make people put more effort into their posts in the long run so it'll hopefully be a good thing.