Actually it is quite common to buy weed laced with SOMETHING, depending on what it is laced with it can either get the user addicted (so you have a repeat customer) or gives the user such an enjoyable experience that the user buys from you again (so you have a repeat customer).
There is where they end up profiting, not wasting money
The first time I smoked weed, I got a bit of blurred vision but that was about it. It didn't even make me feel high, I was pissed (and so were my friends that were smoking with me).
You just gotta find a consistently good dealer like my Grandma has.