They didn't before? It's a credit like any other type, increasing debt burden like any credit. Seems like corru... ahem, lobbying of course. :)
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The reason is that your credit score is impacted by both your available credit (higher is better) and credit utilization (lower is better). When you pay of the last of a home loan and close that account, your available credit goes down and your credit utilization goes up (assuming you had any other debt). Both of those hurt you. When you pay the credit card down to $0 and leave it open, your available crediot stays the same and your utilization goes down.
It can have a short term impact, sure, but long term (a few years), your score will be fine. I paid my last student loans years ago and I did see a sudden score drop and not I'm sitting right below 800 with no debt.