That picture was taken today at an anti-immigration reform protest at a
shopping mall high school in California.
Pass HR 4437.
(Pictures of another protest here.)
UPDATE: A thought: You can fly whatever flag you want to at home or during cultural events like May fifth. That's your right. But this trick and the use of Mexican flags at political protests kind of like Japanese planting their flag on Dokdo. You should expect that it will not go down well with the natives.