If you let your jars sit long enough, the "pop top" will actually collapse in, just as if you had boiled them. It is the moisture sucking the vacuum seal tight.

So in essence, jarring your tobacco is 10000000 times more safe than leaving it in a tin. Also, I have never open a mason jar that I put tobacco in to find it bone dry and ruined...I have opened a tin that I had forgotten that I opened and found it more dry than desert sand...jars for the win.