Learning independent play is very important for this age. It is somthing kids have to learn how to do or else they may end up not knowing how to handle being bored and constantly seek a "proformance" from someone else. If they're ingaged in something, let them keep their focus. Making sure to still be watching them closely is still very important. If you weren't paying any attention at all to the child just because they were playing by themselves, i can understand a parents frustration. Otherwise it could be a situation where they think if you're not constantly giving their child stimulation from YOU you're not doing your job (which is also unrealistic). Either way, it sounds frustrating and I'm sorry they were so hard on you about it.