As a researcher, I support paying research participants so that all costs are covered and there is a little left over as an appreciation for their time, regardless of whether it's for drug research or other health/social research.
However the idea that there would be no participants if we didn't pay is wrong. The thing is that you would get a much less representative group of drug users if you didn't pay for a study like this one, given it requires a good bit of your time and likely travel time and costs around it. You would still get people who are just genuinely interested in helping research or who just like to chat about drugs! but this wouldn't be as broad a group.
Another example where people contribute without payment is all the online surveys people are doing at the moment. Of course they take less effort and less time out of someone's day, but people still generally do them even though they get no payment.
All in all, I think the 'anti-crime lobbyist' in this article is quite misinformed. We are never going to be able to help people and know what is going on unless we engage with the people who are doing it, it's that simple really!