Because if you click it normally, you will only sleep on the sofa if you have a sofa. Ergo, no additional server hits if there's no sofa. Potential hits reaching the sofa if you have it unchecked and do it manually instead, but no more than any normal people not using mafia would have.
And yes, mafia COULD uncheck boxes and the like... but that would leave people who are in a sofa-clan 99% of the time possibly losing their main form of restoration if they happen to restore MP for ode in a non-standard clan. If you don't want it to use it, uncheck it. It's really quite easy...
Though there is one potential 'change' that might be good, if it doesn't happen currently. Mafia should probably go into an abort state (stop automation, red background) if you try to restore in a sofa without having one. So that it doesn't keep trying to heal forever using that. Though it may already do that... if so, then ignoring the background and the message that you don't have a sofa (by seeing that it's causing you damage) is on you.