I think this comes off a bit too strong (as well as the replies to this to be fair)

The example isn't quite accurate. If a friend bought you lunch, the social norm of reciprocity would incline you towards buying them lunch in the future (i.e part of your paycheck)

Free open source software is a public good. While there is no obligation to give back, giving back helps that public good become more useful to other people (including your future self). I'm against making contribution an obligation, but I'm not against light social pressure upon philanthropists who have the means (which is what the parent comment was doing).