Yes, it is legal the ADA states that they may not disriminate against employees with disabilities, but there is nothing discriminatory in asking that an employee who may need to leave the floor more often than most to let her supervisor know where she is that is a reasonable business request and one that does not interfere with the reasonable accommodation being allowed to leave to do these things that your employer is allowing you. You may find it annoying or absurd, but that does itself make this discriminatory. All your supervisor is doing is asking to know where you are, which is reasonable.