The problem is that it's free. Whenever the client has a problem of any kind with their software, this allows them to toss it at you immediately, at no cost to them whatsoever. Charging a fee, even if it isn't a very high fee, would probably make them think twice about doing this... it would encourage them to look at their problem for a few minutes at least, to see if they can't solve it themselves, before they decide it to hand it over to you and incur costs.