Reply from United:-

I have verified the U.S. State Department's website to see the entry
requirements for Aruba. It is mentioned that "In addition visitors to
Aruba may be asked to show onward/return tickets, proof of sufficient
funds and proof of lodging accommodations for their stay. Length of
stay for U.S. citizens is granted for thirty days and may be extended to
180 days by the office of immigration." I trust in view of the above
mentioned information, you will agree that our agent at Buffalo was
justified in asking for you to make the necessary change in your
reservation to travel. You may visit the following link to obtain
complete information regarding the same: