I have also heard of this quite alot, but i personally think it is just "skunk" that causes that, or most likely all the shit that is mixed with it, As you probably know there is hash in the uk prolly the cheapest cannabis you can buy, but it is mixed with so much shit its insane, and all the stuff mixed with cannabis that probably causes these side affects. When i was younger i bought a half ounce of hash and when i brought it home to make a nice joint i was half way through and i ended up pulling a big peice of plastic (from an asda bag or somthing) out of it and it just fell apart, this plastic must have been about 10cm long and there was loads of it all burnt up and bubbling when i burnt the hash. I have also heard of glass on weed to look like trichomes and all sorts of shit.
If it was just legalized and regulated it would be fine, everyone could grow their own and/or buy it from shops and know it wasn't cut with all these harmfull chemicals. I could say that for every drug there is, there would be less deaths and crime if the government just stole it all and produced "clean" drugs. And imagine how much that would fuck up cartels and international drug smugglers. The government would finally cut drug related crime in half, if not more.
I know you might think it would introduce innocent people into hard drugs, but if somone is going to do heroin or blow they are going to do it legal or not todays society proves it.