Maybe it is cya type thing to avoid a lawsuit. Towel bar manufacturers are concerned someone will use them as a handicap grab bar for which they are not designed. Yours is apparently substantial but not designed as a grab bar but someone may try to do so. Avoiding screwing into a stud would allow it to be pulled out of the wall before the pin of the towel ring broke under the weight of someone pulling on it half hanging above the tub resulting in a serious injury and the company being sued. Do the instructions say anything about not using it as a grab bar? Bet they do.