That aphorism assumes some things about a person's goals in life that don't hold for everybody.

I have 12+ years of engineering experience and would do it for free given it provided valuable experience or connections that I couldn't obtain otherwise

You should write Chicken Soup for the Designer's Soul-- just don't do it for free.

Seriously though, as easy as it is to post cliches, can't you imagine situations where you should work for free? Like in the situation OP is in, a foreign national who can't accept payment for work in the US?

Agreed, though this is perhaps an exception. Its obviously a lifestyle choice rather than a long-term strategy:

>I will work with each startup up to 1 week and then move to another city/startup.

Unless you are batman.