If you used paid time off (like paid sick leave) which you had earned for the absence, they cannot penalize you for doing so: an employer may not penalize employees for using benefits they earned, since to do so is to effectively deny the employee some of their compensation. But without using PTO, then an employer could discipline you (such as in this way) for an absence: the law does not, except for the use of PTO or when an employee is eligible for an uses FMLA leave (you can find the rules for FMLA leave on the U.S. Dept. of Labor or DOL website), let employees miss work, even for illness or with a doctor's note. If you do miss work without using PTO, FMLA leave, or with your employer's express permission to do so, you have an unauthorized absence and may be disciplined.