Legal vs illegal dealers are two different things.
A legal dealer/grower is expected to follow certain laws and provide a safe product. In return for a high demand product they have to follow licensing rules and have a safe product.
An illegal dealer has the incentive to make as much money as fast as possible because of the risk of future prosecution. It is more beneficial for the illegal dealer to switch you from $100 of weed per month to $2000 worth of heroin per month. It has a much worse health outcome for you, but a better financial outcome for them.
They might have that incentive, but there is a reputation aspect I think you're ignoring. Most illegal marijuana dealers I've met were just interested in selling weed to anyone who wanted to buy it to pay rent, tuition, or support their own use. They operated under fairly low margins for a small business and essentially didn't price in the legal risk. One guy I know only made about $10-12k/yr in profit.
A legal dealer/grower is expected to follow certain laws and provide a safe product. In return for a high demand product they have to follow licensing rules and have a safe product.
An illegal dealer has the incentive to make as much money as fast as possible because of the risk of future prosecution. It is more beneficial for the illegal dealer to switch you from $100 of weed per month to $2000 worth of heroin per month. It has a much worse health outcome for you, but a better financial outcome for them.