There are four basic lines of argument:
- legal. It's against the law; there are strong 1st and 4th arguments against surveillance, and 5th and 6th amendment arguments as well. Some people have even tried to argue that if encryption is a munition the 2nd amendment applies. The "we could stop bad people" argument applies especially to the 4th.
Similar legal protections exist in Europe, although not generally as strong.
- practical: either a system is secure or it isn't. Handing it over to anyone increases the risk of compromise. Bans on effective encryption are self-defeating.
- collateral damage: US intelligence agencies have a track record of killing innocent people themselves (e.g. drone strikes), supporting murderous governments (CIA in south america), funding terrorism and failing to prosecute the guilty (Iran-contra), use of intelligence for domestic immoral politics (Watergate, FBI vs MLK). Handing over data about your e.g. Chilean users to the CIA may result in them being murdered.
- international hypocrisy: saying that mass surveillance is OK says it's OK for other governments too. Do you support Chinese interception against their adversaries? Are you happy to turn over your entire email history on entering a country?
- finally, I'm going to question how much it does help. The Paris terrorists were known to the police, as was the killer of Lee Rigby. Intercept evidence is generally not used in trials. Nobody is presenting a cache of encrypted emails found on a computer used by the Parisian terrorists and saying "if we'd decrypted this we could have prevented it".