If I remember correctly, BPA is used specifically to make plastics clear and tough (specifically polycarbonates, e.g. the classic Nalgene water bottles) and tends to leach mostly in the presence of heat or acidity. No BPA in baggie plastic. With Ziploc baggies, the major issue for pipe tobacco storage is that they're gas-permeable and thus tobacco stored in them will dry out over time.
As for pipe tobacco and humidity, I live in a fairly dry climate (Colorado) and have not had too many problems with drying out tobacco over the relatively short-term (2 years at this point) that I've been smoking. Most of my storage jars are either Ball canning jars (for long-term storage) or bail-top jars with silicone seals that I got at IKEA (for short-term storage of blends I'm smoking). European-style tins that are closed will stay nicely sealed for decades, and I have yet to purchase any tins not in that sort of container. I have some smaller jars that don't seal all that well for the last ounce or two of a blend, and I've had one or two get slightly crunchy, but those blends actually benefited from some drying time, IMHO. I have yet to use any of the hydrating disks or other methods you'll find digging around in the archives here...
I suppose that with a wooden tobacco jar that has no silicone seal, a humidity disk might not be a bad idea, but I'll let the experts weigh in on that. :D