It's not light coming through the backing paper but the ink on the backing paper affecting the emulsion of the wrap of film above.

This has recently been discussed:

Kodak needs your support and they make the world's best film, bar none - why not buy their film?

Lomography only sells crap film because they buy it for nothing and can sell it at outrageous prices to fools who don't know better. Pretty soon the numbers on the negative will be 'hipster cool'.