I think it's different when it's a sitting President, who is their boss, and "just a protectee" which is now as a former POTUS and candidate. A sitting President being protected isn't just about his own safety, it's the nuclear codes and other sensitive info. That is also their repsonsibility.
My other guess at this is that the USSS sets up the perimeter and is responsible for that. They are not responsible for the logistics, where the stage is set up, etc. Areas outside of the perimeter... they can't guarantee anything.
But there's enough there for a lawyer to think there's a case. If I got to a Tigers game and get shot by someone.... I can sue the hosts of the event. In this scenario Trump's campaign was the host.
I worked security for the beaitifucation mass of Fr. Solanus that was held here about 7 years ago. It was at Ford Field. It was a global ceremony and a huge deal. The biggest security event held in that building since the Super Bowl. The Archdiocese was responsible for security even though the FBI and Homeland Security were involved as well.