Wouldn't it be more practical to invent a glass that can change colour when a date rape drug is detected, then stock those in as many bars as possible
(Of course, I don't have a degree in Chemistry or anything, so I have no idea how feasible this idea is)
Well, for one thing, you're going to have to have some process available to reverse the reaction as well, or else the glass will just be tinted forever, which would make it kinda useless as a date-rape prevention glass. But the biggest concern would probably be money. A glass like this would inherently be
significantly more expensive than your usual glass, and bars, like all businesses, like to make as much money as possible. Announcing they sell date rape glasses isn't likely to significantly increase their profits (though in an ideal world, it wouldn't lower them either), so they end up losing money on the date-rape glasses.
Not announcing that they have these glasses would be even worse from a financial standpoint, because then they're spending extra money on something the customers will never know about.
It's all about the money.