Oh they can climb just fine...as youngsters. You just don't want to give a 1000g+ snake with a body as thick as a can the opportunity to try it and end up falling on something.

I have two little ones that love to climb. My spider girl lives up to her morph name, as she is all over her tank every night, climbing over her plants and tree. My male pastel, despite the kinked back, still loves to climb...tho he is slowing down now that he's approaching 500g.

The 'myth' is true - BPs are not good climbers. It's just that no one told the babies that.
Just look at bears: the cubs are awesome climbers, but the adults never even try. They just get to a point where they're too clumsy and heavy to do it anymore.