"But terrorists will just ignore the law" is a bad argument. It ignores the fact that the ban still increases the power of the government to act against terrorists. The problem with the law isn't that it's useless against terrorists. The problem is that it also increases the power of the government against the innocent, and possibly the power of some non-governmental criminals as well (depending on implementation).
A problem could be that people in general don't care about: "increases the power of the government against the innocent" -- because that's a theoretical problem in the "distant" future?
Personally I think the US gov getting too much power and gradually in small steps get access to "everyone's" communication, is like 1000 times more dangerous than "encrypted terrorism messages",
and, what can be a good way to make people in general better understand the risks & dangers with small steps towards a dictatorship? Maybe if everyone reads 1984 :- / or if everyone was better educated about what happens if you're in China an critizise the government there?
I didn't mean literally all lawmakers etc (although that's what I wrote). The next time I can try to write "some of them" instead.