What the gov agencies are afraid of
What the gov agencies are afraid of is the near-real-time disclosure of a NSL or FISA/FISC letter tipping off the suspects that they were on the (S)hit list of surveillance.
I suspect, tho, that in reality, the agents do NOT need our passwords under demand during interrogations. They already have the power to compel Google, MS, Yahoo, and others to turn over direct, remote, invisible/ghosted/parallel, real-time access to our email activities. They already have code that in the form of a few pixels can hijack our sessions and to on an every-logon-of-target upload bits and pieces of our hard drives in the background so as to not spike the CPU. I've since 1995 suspected that criminals could do this, but over the past 10 years been willing to suspect the government more than criminals DO DO, and it probably is how they brought down some of the more resilient but ultimately careless criminal kingpins, not just by microwave lasers and in-room cameras, and sonic/electromagnetic detection of keyboard clicks and CRT hazery-imagery of passwords.
Forcing us to hand over passwords probably obfuscates the existence of huger, greater capabilities, stuff Snowden probably has not even released. Hence, the UK heavy handedness in helping itself to some US intel, and also chummy-cousin-kissings. That just reinforces that ANYone passing through the UK can be called anything it takes to basically steal hard drives and electronics even for non-security reasons, even to economically destroy or psychologically torment very specific people. I suppose people will plan their flights accordingly now, if they even remotely associate with newspapers or speak out on such matters. So, I wonder often whether I will ever set foot on UK soil or territories, or if I have to, just take NO electronics having personal, proprietary, business plans or stuff that just franky isn't any of the UKs effin bizness. OTOH, maybe I'm just momentarily "over the top", but at some point, in the next 5 or so years, "over the top" will be the new norm, the new baseline.