Why do Americans believe that Mexican legal and illegal immigrants don't contribute to the United States?

I don't know about not contributing but I know it pisses people off when Mexicans enter the country illegally, live here without paying taxes. In some cases they take advantage of our schools to educate their children, while again not paying taxes. This country uses taxes to pay for things like roads, education, and public services such as fire and police. In many cases illegal immigrants use these things without contributing towards their upkeep. I'm all for immigration, when it's LEGAL, to insure the immigrants don't just get a free meal ticket. Unfortunately there are people who feel like they can enter the country illegally, make money to support their family, educate their kids in our system, use our roads, use our hospitals and public services, all without paying taxes that are essential for our economy. Many don't give back to the community by not paying taxes, and with the economy in the state it is we need our tax payers.