They can "blow the whistle" to others in Congress or make a public fuss, directly or indirectly. The assertion that they would thereby be breaking the law (being sworn to secrecy) is a circular argument. If they are apprised of an illegal act, they are stuck between conding that illegal act, or "breaking" a law that forbids disclosure. That bottoms out on their sense of what's best - and that calculus can be selfishly personal, risk averse, or aimed at making the situation better "for all."
Why couldn't they have expressed their concerns during the briefings? Why couldn't they have asked for their own legal analysis? It would seem to me that they had multiple options that didn't potentially compromise an effective program or the security of the country.