I don't even think it is about the USSS bowing down to Trump's demands altho they certainly could have.
The USSS most likely has a certain area/perimeter that they are responsible for and that is the ONLY area they are responsible for.
Anything outside that area is NOT the responsibility of the USSS but COULD be the responsibility of the Trump campaign.
Meaning if the Trump campaign wanted to hire additional security beyond the established perimeter, they were welcome to do so.
My guess - based on a 50+ year record of Trump not paying anyone - the campaign chose not to do so.