Medicare is a form of welfare, just branded differently. It's a means-tested benefit funded in a pay-as-you-go manner via income tax just like any other. The means are just different amongst various programs.

Are there any actual cases of making more earned income via a regular job worse than taking that extra dollar of pay? I'm guessing a few very rare corner-cases exist, but I can't immediately think of any. I imagine they would be somewhere in the neighborhood of the EITC or AMT type things.