But these companies do more than what is required by law. They do not by law have to provide API access, only to provide the data in some form. None of the smaller webmail hosts cooperate in PRISM.
And as for warrants, no they do not always need a warrant. They only need that if both parties in the communication are US citizens. If none of them are no warrant is needed at all and if just one party is US then they (according to the Wikipedia article on PRISM) can wiretap for up to a week without getting a warrant.
And as for warrants, no they do not always need a warrant. They only need that if both parties in the communication are US citizens. If none of them are no warrant is needed at all and if just one party is US then they (according to the Wikipedia article on PRISM) can wiretap for up to a week without getting a warrant.