If someone is not working for a long time then officers DO ask how the applicant is supporting himself/herself. This question has nothing to do with public charge; rather it has to do with determining the moral character of the applicant. If someone doesn't have a job then without a reasonable reason/proof/explanation, USCIS automatically presumes that applicant is either working off the table to avoid paying taxes or s/he might have been involved in shady things like illegal gambling, gambling for living, involved in prostitution or drug dealing or something like that. USCIS doesn't need to have a proof to make that determination; rather their presumption will be enough to determine all this for the purpose of judging moral character.
Just two weeks ago I went to INS office with someone in Garden City office in NY. This person NEVER worked in his life since he is living in this country for the last 20 yrs. He told me that he has been supported by his GF and he is actully living off gambling which is legal but not paying money on gambling income is a problem for naturalization and violation of tax laws. Anyway, he has filed taxes all these years though wherein he showed his income $385/a year. He said that he filed taxes just for the sake of filing it, and the reason he showed this much income because if he had claimed making of $400 or more in a year as a self-employed then he would be liable to pay taxes to the Uncle Sam.
During the interview, officer asked him to provide 5 years of tax-returns. Officer went to check line-by-line on his tax returns and jumped off the chair when she saw that he made only $385/a year. She asked him three times if he only made this much money in a year for 5 years. He answered yes. Then she asked him if he is getting any public aid from the govt. which to he said no. She asked him to explain how he has been supporting himself all these years with this much income. He said that his GF is supporting him. Officer asked him to provide a notarized affidavit from his GF saying that along with her income tax returns which he didn't have during the interview. His case put on continued. He now has big problem because his GF is illegal who doesn't file tax returns nor she pays any taxes because she gets paid under the table. But she does support him.
I've witnessed this so many times. Some officers don't care while others do. Tax returns are asked by USCIS not for the purpose of finding whether someone has paid required taxes or not; rather it's asked for bunch of reasons.
