Trump can say whatever he wants at his rallys. They are his rallys after all. His supporters come to hear his message and see if he's worthy of their vote. Without distractions from the haters, they could decide they don't like the guy after all.
The Bill of Rights says differently. Not sure if you lived through the 60's, but I can't imagine people not standing up to the Civil Rights injustices and just saying, let the Racists and Bigots have their rallies don't protest.
Trump's Free Speech is not being infringed upon despite what he says. People protesting have the right to assemble and exercise their Freedom of Speech as well.
Assembly
Assembly is the only right in the First Amendment that requires more than a lone individual for its exercise. One can speak alone; one cannot assemble alone. Moreover, while some assemblies occur spontaneously, most do not. For this reason, the assembly right extends to preparatory activity leading up to the physical act of assembling, protections later recognized by the Supreme Court as a distinct right of association, which does not appear in the text of the First Amendment.
The right of assembly often involves non-verbal communication (including the message conveyed by the very existence of the group). A demonstration, picket-line, or parade conveys more than the words on a placard or the chants of the crowd. Assembly is, moreover, truly free, since it allows individuals to engage in mass communication powered solely by sweat equity.
The right to assemble has been a crucial legal and cultural protection for dissenting and unorthodox groups. The Democratic-Republican Societies, suffragists, abolitionists, religious organizations, labor activists, and civil rights groups have all invoked the right to assemble in protest against prevailing norms. When the Supreme Court extended the right of assembly beyond the federal government to the states in its unanimous 1937 decision, De Jonge v. Oregon, it recognized that the right of peaceable assembly is a right cognate to those of free speech and free press and is equally fundamental.
The right of assembly gained particular prominence in tributes to the Bill of Rights as the United States entered the Second World War. Eminent twentieth-century Americans, including Dorothy Thompson, Zechariah Chafee, Louis Brandeis, John Dewey, Orson Welles, and Eleanor Roosevelt, all emphasized the significance of the assembly right. At a time when civil liberties were at the forefront of public consciousness, assembly figured prominently as one of the original Four Freedoms (along with speech, press, and religion). When, however, President Franklin Delano Roosevelt switched to a different grouping of four freedoms in an effort to rally support for American entry into WWII, assembly (and press) dropped out. Neglect of assembly as a freestanding right has continued ever since. In fact, the Supreme Court has not decided a case explicitly on free assembly grounds in over thirty years. But despite its recent state of hibernation, the freedom to assemble peaceably remains integral to what Justice Robert Jackson once called the right to differ.
