It is not the same plastic. They are really water bottles, HDPE not PET
If you are not one of those obsessed with the idea that it will leach chemicals and kill you it should be OK. I needed some 3 gallon ones and bought 2 of them. I would not do extended periods of aging in them though. I do about 2 weeks then package.
You will probably get some responses to NEVER ferment in them!
I use disposable plastic carboys from Menard's. They are PET plastic, and they don't have any hard-to-clean handles. I haven't bought one in a while, but I think they are about $5.50 for 4 gallons of "spring water", and no deposit on the carboy because you're supposed to crush and recycle them.
My biggest objection to the Walmart water bottles is trying to clean the handle areas. Maybe it's not a problem, I haven't tried.