Ask HN: Is free-for-commercial-use a viable option for SaaS applications?

If you want to offer groups and organizations that fall into the not-for-profit category a free version of your application, how can you prevent for-profit organizations from claiming to be not-for-profit and there by bypassing any costs?

I know of a few companies that have used Google Apps without paying anything - simply signing up as a non-profit organization.

Is this maybe not a concern at all?