Just because someone pays some taxes (it's hard to avoid paying sales tax if you buy a thing in a store, regardless of your citizenship), doesn't mean they're a net economic benefit to the country they live in, depending on exactly what taxes they pay and what taxpayer-funded services they get.

Immigrants come to the US with a variety of financial situations. Some immigrants are blatantly scamming welfare systems designed for poor Americans or are outright committing fraud (this is much of what is going on with the federal investigation and charges of Somalis in Minnesota). Other immigrants are paying taxes comparable to what American citizens would pay (this is probably the case for most people on H1B visas in the tech industry).