The other accounts need voting power. In order to get voting power they need to invest. When someone invests into these bots, they invest into STEEM. This grows the STEEM economy. If they choose to sign up for free accounts to vote some dust on another account, it is not going to make a difference. The bot owners would be really working hard for dust. It would just be easier to blog or comment occasionally and get some votes that are worth more than dust.