Senators are legislators. Legislators write laws, changing things from "illegal" to "legal" or vice versa. The question of whether conduct should be illegal is thus absolutely their business, and the purpose of senate hearings (aside from grandstanding) is to give a public forum of record where these issues can be discussed to guide future legislation.

There seems to be a strange misperception that judges should decide whether something should be legal or not, when their role is to /interpret/ the law as it exists, and /apply/ the law to specific situations.