Mexicans wave Mexican flags when invading the US and demanding rights? Shouldn't these oh so poor people be..?

waving American flags? Afterall they are moving to a different country, one would think that they would support the USA and not stomp on or burn its flag like so many do (search illegals burn flag on youtube) Also if Mexicans are so poor why is Mexico one of the most obese nations in the world? Surely Mexicanos must be eating alot, yes?