First, we offer paid upgrades for features like custom domain names, and automatic mailing list signup forms. We're also hard at work designing new products for missionaries and others in Christian ministry. Income from those upgrades and other products will help to cover the expenses involved in hosting the Free* missionary websites.
Second, by requiring frequent updates on missionary websites, we make sure we're only incurring expenses for websites that are adding value. If a missionary is willing to invest the time in keeping the website up-to-date or to invest the money for an upgrade, then it's a website we believe is worth investing in.