I guess there are several answers to that, a few of which are:

Much the same could be said of the first use by someone of tobacco, or even alcohol.

Not everyone who tries heroin gets addicted and I don't know of any reliable research results on the number of those who get hooked versus the number who don't. Speaking anecdotally, I have known people who have tried heroin once or twice and didn't use it again and a number of others whose use was controlled, confined to weekends or other planned occasions. There is such a thing as an addictive personality; a lot of heroin junkies who manage to kick just become horrible alcoholics instead. If you're not an addictive type, simply trying out heroin won't make you one.

People who are trying heroin for the first time are seldom thinking in a cold and rational manner, much like those trying the aforementioned tobacco and alcohol, not to mention cocaine, acid, sex, downers, crank etc. They tend to be young and in the midst of an intense exploratory stage of life. They're testing their limits and that means pushing them.

This isn't to say I approve of any form of heroin use (needles creep me out, for one thing). It's obvious that the model of a single use inevitably leading to addiction is false, though.