Dimmer switch remains powered with switch off!
Dimmers work differently

To start with, a dimmer is a smart device, and all smart devices need power themselves for their own internal functioning.   

How do they get power?  That's kind of a problem.  A plain switch loop doesn't need neutral - it gets always-hot and delivers switched-hot.  So many older switch loops are wired with no neutral.   However, old style incandescent bulbs have a unique property: when they are not lit, their resistance is near zero.  So many smart devices (and almost all older ones) power themselves by leaking power through the incandescent bulb.  CFL and LED bulbs don't play well with this, unless they're designed to. 

How do newer smart devices power themselves? 


Some newer still do it this way, because they're afraid of having a lot of returns from consumers who find out they don't have a neutral.  
Some simply require a neutral.  
Some misuse the ground wire as a neutral, and they don't blow GFCIs because they flow very little power.  (How they make it through UL listing is a mystery to me.) 


Dimmers which require or steal a neutral should not put any power on the line when "off", but you should not count on that for safety! 

A dimmer without a neutral should, and must, energize the switched-hot at all times.   

Some dimmers have a hard-switched "off" (and sometimes a hard-switched "on")... but it's past a stiff "detent" and people never push the switch hard enough to realize the detent is there.