Police must rely on evidence; they can't just arrest people indiscriminately.
Tips
memory
皂 means 'black' (think 肥皂 'soap', traditionally dark). The four colors 青红皂白 (green, red, black, white) are a classical shorthand for 'right and wrong, this and that' — refusing to tell them apart means acting blindly.
grammar
Almost always used adverbially with 地 before a verb of judgment, accusation, or punishment (骂, 打, 抓, 处理).
