Probably the most common objection to ending the prohibition on cannabis and creating a legal, regulated market is that kids are no longer smoking your dad's pot, they're smoking super-strength skunk. But it's that very system of prohibition that's lead to the creation of stronger, dangerous strains.
It's all down to something called the Alchian-Allen effect. Say you've got two similar but not identical goods e.g. pricey top quality Pink Lady apples and cheap bog-standard Cox apples. If there was an identical increase in price on every apple sold, say a pesky politician levied a 10p apple surchage, you'd see a rise in demand for Pink Lady apples and a fall in demand for Cox apples.
Why is this? Well, while both products have seen identical absolute price increases, the high-grade Pink Lady apples have increased by much less in relative terms. Before the apple surcharge, a Pink Lady apple might have cost more than twice as much as a Cox apple. After the surchage that differential will have shrunk substantially. So in relative terms, Pink Lady apples are now much more attractive to consumers.
How does this apply to weed I hear you asking? Think of different strains of weed just like Apples. You've got the pricier high-strength skunk and other weaker-strains on offer. If the relative price of one went up, all things being equal demand for the other would rise.
We should think of cannabis prohibition as the equivalent of an across the board fixed increase in price. The law doesn't distinguish between different strains of weed, you're given the same sentence whether you're dealing skunk or weaker, safer varieties. Dealers price in the cost of getting caught and going to prison, if that costs the same regardless of how strong the product is, then you'll see the price differential between skunk and weaker strains falls, making skunk the more attractive product.