Mark Smith well, i didn't say I reply instantly but usually I always reply. To me it's about manners I'm raised like that, but maybe it's because I worked in a lot of different jobs so I tend to think about their perspective.
But that's just me, maybe the are used to it like Isaac Weber mentions. It doesn't matter to me, I will try to always answer a question and even most of the time it's just a 'thank you for considering me, I'm currently busy.' or 'I am not doing XYZ' .... oh ... but I use a specific email for almost every service so .... I always know where they find me and usually I put a 'currently not looking'-sign out there :) .... And the 2-3 recruiters who still write I answer ...