The media and public in America see the existence of an underground metahuman organization as basically unnecessary and dangerous. Mutants already have "more legal protection than they need or deserve" in the conventional wisdom.
However, there are several facts known to Underground organizers and experienced Underground members that are not known to the general public. For example: the FBI uses surveillance techniques to build dossiers on the lives of mutants and sometimes uses those dossiers to blackmail them - sometimes "officially", sometimes simply for the benefit of corrupt agents with access to the files.
Collectively, the members of the Underground have little faith in the 14th Amendment to indefinitely protect mutants. Eventually there will be either a legally sufficient excuse to put discriminatory policies into practice (such as a major Supremas action, an independent mutant crime spree, or a broad quarantine law), or a radical legal dysfunction (such as a judicial decision that takes the Humanity's Protectors' argument that "they aren't really human" seriously and sets legally convenient precedent that mutants are not persons under the meaning of the 14th Amendment). The continuing clumsy attempts to regulate and control mutants at the local level makes the Underground believe their situation is extremely precarious. (They are likely correct.)