>We post regularly online without any expectation of payment but then we never considered that the output could or would be used for commercial purpose.
We did. If you are posting here, that means you've explicitly stated that you read and agreed to Y Combinator's TOS. Which include this clause:
> you hereby grant and will grant Y Combinator and its affiliated companies a nonexclusive, worldwide, royalty free, fully paid up, transferable, sublicensable, perpetual, irrevocable license to copy, display, upload, perform, distribute, store, modify and otherwise use your User Content for any Y Combinator-related purpose in any form, medium or technology now known or later developed
And people are still fine with this apparently, otherwise they wouldn't use the service anymore.