Congress shall make no law [...] prohibiting the right of the people peaceably to assemble

Note that the 1st amendment doesn't say "peaceably protest", it says "peaceably assemble". We have as much right to assemble to celebrate our nations founding as any protest. Equal justice under the law demands that if Newsom allows these people to gather, then he cannot legally bar similar gatherings on the 4th. He cannot claim one gathering is safe and another is not, he does not have the legal authority to dictate such things.