I don't know how much BS it is, but GPAs can vary so much, based on where you went to school (I hear Chicago is very challenging) and what you majored in, whereas LSATs are completely standardized. That might be why some law schools put more weight on LSAT scores and your PS than GPA.
As for indicating success in law school and beyond, I doubt that. I think a good LSAT score indicates a.) being able to control nerves under pressure b.) being able to think quickly and c.) being a good test taker. Some of that might be useful in practicing law and going to law school, but is only a small part of the whole process. I.e., the LSAT doesn't test people skills!