This is because we get a hit to our server even if the screener is off, and it creates massive traffic for us cumulatively. As of now, that's about 480 million page views per month, which takes a fair amount of server infrastructure to support, and that costs money. So voila, that's how the pricing plan is meant to work - if you send us a ton of traffic load, you'll have to pay more, and if you don't, you don't have to pay much at all.
More detail: Any page where you place ethnio code tracks the number of views to that page, and counts as a pageview every time someone loads that page. It still counts even if your ethnio screener is turned "OFF".