What do you think is the business of senators?

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.

They literally set the policy for the nation, they arent Police. It is percicely their business.

If it was truly about policy they would have accepted the lawyer, it's theatrics and grandstanding and campaigning from that dais.

If a US DoD vendor is getting cozy with a perceived enemy to violate human rights it is 100% the Senate's business.

Define "getting cozy" in this context and please explain why a public hearing is in any way better than a private one or even just exchanging correspondence.

Nothing Facebook or Cambridge Analytica did regarding user data was illegal. Please clarify how the matter of legality determines whether a Congressional hearing is unwarranted, or undemocratic.

I don't think it was warranted in that case either, it was an exicise in public humiliation, a bunch of people who never created anything getting off on talking down to someone much better than them.