No need to paint the male gender as a whole as being filled with sociopaths. It's just the law of large numbers at work. There's maybe 30 million American men in the age rage that are likely to pick up srange women; if just 1/10 % of them are sociopathic predators that's 30,000 predators; and since they *are* predators they'll be overrepresented in young women's encounters with men in pick-up scenarios. Small numbers can produce disproportionate problems. In this case it represents numbers the actions of such a small proportion of men that our ideas about how normal people act aren't a reliable guide.
Drink spiking is a very rare crime. Most studies that look for evidence of it find very little. The highest I found was a government study which found date rape drugs in 4.5% of the cases from four sexual assault clinics. Note this is 4.5% of the cases where the assault occurred, so we're not talking about 4.5% of encounters, we're talking 4.5% of rapes. 4.5% is certainly high enough to be a concern in certain situations, like residential parties at a college. In such a situation a date rape drug detector might actually have some utility, even though it addresses relatively rare actions by a tiny proportion of men.
A bigger concern than what we think of as a "date rape drug" is alcohol itself. The same study that found date rape drugs in 4.5% of sexual assault samples found alcohol in 55%. This result is consistently found across studies: alcohol is very frequently associated with sexual assault -- around half of the time. This is especially concerning because some people (men and women both) don't believe that surreptitiously incapacitating someone with alcohol in order to have sex is rape. They don't distinguish ethically between two people getting drunk and having sex and one of them slipping extra alcohol into a drink.
But the fact remains most men wouldn't do something like that. But that doesn't preclude the possibility that a woman might often encounter the few remaining men who would. A typical man has sex with a small number of women many times; a man who has sex with a large number of women only once is bound to be encountered by women disproportionately often.