Yes. Yes, they need cause, but not in the Fourth Amendment definition. The cause is you've requested to enter the US or you've been found hanging round the vicinity of an airport, land border or navigable waterway.

My phone has been also searched.

The kindly asked me to hand over my unlocked phone after they asked me to show that I have booked accommodation which I did on a phone. They asked who are these people on emails, then handed the phone back.

They only quickly browsed through my emails though.

My recommendation is that everyone prints on the evidence of the purpose of their trips beforehand: booked hotels and a list of conference they plan to attend (plans may change, however).

In the US the Bill of Rights and most other things only apply to citizens (and maybe lawful residents).

CBP agents have broader authority to conduct searches of non-citizens and their belongings at ports of entry without a warrant, including devices like laptops and phones, to determine admissibility to the United States. Searches of citizens generally require a higher standard of suspicion or a warrant.

In the past, someone has taken a flight between Canada and Mexico while under suspicion of the US authorities, and the plane was forced to land and the person forcefully detained, just because they entered US airspace.

Just losing your ESTA over weed (federally a schedule 1 narcotic) is a fairly small punishment considering the CBP enforces federal law.