Cause glock doesn't have normal rifling, and don't follow the same rules as everyone else. Call em and see what they say about it and come back on here and let us know.
Been a few mishaps with cast and glocks. I won't load cast in mine either.
I do run a bronze brush with some solvent then a dry patch then a lightly oiled patch between every 100 shots or so, but i also do that with all my guns I shoot cast lead through, just as a precaution for lead build up. Havent had any problems yet.