Is it illegal to stain hotel towels?

Not illegal. The hotel's towels are used to provide cleaning services to guests. Any stains can be dealt with by the hotel staff in charge, and there will be no claim for compensation or any violation of law. Towel is a kind of cleaning product, which is characterized by square pure cotton textile. The method of use is to wet it and then wring it out to wipe the skin to achieve the effect of removing stains, cleaning and cooling.