Touches that fall inside of a desktop widget are considered part of Notification Center, which is set up to have a "one finger press" gesture do a normal click and drag.
The intent is for users to be able to rearrange widgets, since normally in the Notification Center dragging with one finger would scroll it rather than doing a click and drag, and this way it works somewhat similarly to iOS.
If a right click is needed, either change the "One finger press" gesture in Notification Center to right click, or use a different gesture, such as "Press and tap" to perform the right click.