The businesses they run pay taxes, including payroll tax.
The people they employ pay income tax.
Everything they buy, every contractor they hire, generates economic activity that is taxable, either directly or indirectly.
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But that isn't all.
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Study after study shows that, statistically, illegal aliens are more law-abiding than native-born US citizens:  They live in constant fear of being deported, so they take very few chances on breaking the law.
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Every person they employ, directly or indirectly, is someone who is not on social assistance, and who is adding to the pot, not taking out.
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If the US were genuinely interested in reducing "criminality", taking realistic steps to reduce the 30,000 firearms deaths per year would be a much more logical place to start.
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Except facing that truth would upset the low-information, red-necked, old, white, male, evangelical Christian voters that define the Republican Party.