Enforcing the restriction is very much forcibly preventing someone from committing suicide in their preferred manner (which I'll add before it comes up, is not a manner which infringes on others).
Well, OK, and the fact that certain drugs are controlled means I can't just purchase them and kill myself painlessly, and so on. But perhaps having some drugs only available by prescription offends your ethics too? You must, of course, speak for yourself, but if I were to choose between suicide by gun and suicide by painless drug cocktail, I'd take the drugs every day (well, you know - just the once, actually).
If I take the numbers at face value, it suggests that when handguns need a cooling-off period, some people don't rush out and buy a handgun while in an emotional state, but have a chance to cool off and decide that actually they don't want to kill themselves. I can't see preventing someone from making an emotional mistake as a bad thing.