charro (in a Mexican union context): sellout

I've posted before about sindicatos charros in Mexico being yellow dog unions.

But I ran into another Mexican use of the term here in the fabulous copy left Rini Templeton Art archive with this image, that came with the following description:

Over 5,000 workers at Mexico City's Universidad Autónoma Metropolitana (SITUAM) struggled against wage ceilings imposed by the government as part of an austerity program arranged with the International Monetary Fund (IMF). They also advocated progressive academic policies and solidarity with struggles for democracy. "Huelga Huelga Huelga/Charro el que se raje"/Strike, strike, strike/Whoever gives in is a sellout. Rini worked closely with them.