There are companies that do produce it with chemicals. As well, people often use scrap lumber or donated lumber, lumber found in barns, etc. These still contain the chemicals used to originally treat the wood.
I am unconvinced that the chemicals do not leach out over time. Even if they don't, cutting the wood can still release dangerous chemicals into the air. To be on the safe side, I just don't recommend it. Even if it has a limited spread, some plants have extensive root systems. Getting untreated wood or some of the more modern pseudo-wood concerete or plastic is preferential to me. I have seen a few studies where the salt treated wood leached easily and I wouldn't want to put that into my soil either. Granted, I dont know the difference in processing for the salt treated wood studies.
Animals which decide to chew or pick on the wood can become ill and I am also unconvinced that insects would not pick up the chemicals after having buried into it. Those insects go all over being eaten by other creatures as well. If the pressure treated lumber is treated with dangerous toxins and after being taken apart winds up in a creek or off to one side of the property, there is a possibility of environmental damage.
I try to look at all the possibiltiies because when I look at just one spot, even over a period of years, I cannot see everything that is possible just from looking at that one bedding box. I try to consider the ramifications of interactions throughout the whole system of the soil, the yard, and anything that might interact with it. Paranoid? perhaps..but I think of it more as being as cautious as possible..
