Sounds like a case of miscommunication between you and your uncle. Although it sounds as if he gave you permission to edit his tree, and therefore implied that he wanted you to make edits, it would have been courteous to contact him directly and find out his goals for that particular tree before you make any changes.
Was the tree intended to focus on your your uncle's ancestry or his entire network of relations?
For example, I have multiple published trees. In order to keep my trees manageable, and to help with tracing DNA matches, I have separate ancestry trees for my mother and father, including bare bones ones that show only my direct ancestors and direct descendants.
If one of my nieces or nephews added someone who wasn't a biological relation to one of my DNA research trees, it would not be helpful.
I don't know if I would be mad though. I would be happy to see that they are interested in genealogy and help them build their own tree.