ARNOLDS PARK, Iowa — For almost 40 years, Carole Hinders has dished out Mexican specialties at her modest cash-only restaurant. For just as long, she deposited the earnings at a small bank branch a block away — until last year, when two tax agents knocked on her door and informed her that they had seized her funds, almost $33,000. The Internal Revenue Service agents did not accuse Ms. Hinders of money laundering or cheating on her taxes — in fact, she has not been charged with any crime. Instead, the money was seized solely because she had deposited less than...