I know WOTC is calling it a "conversion clause". However, it appears if Green Ronin, who provides Freeport support for True20, Savage Worlds, couldn't make a single Freeport product for 4e that is different from the True20 and Savage World products without having to give up supporting the True20 and Savage World versions. I imagine that, when the d20 GSL is released, the same would hold true for RPGObjects's "Blood and [x]" line which not only suppports d2O Modern, but also True20 with the True20 version of Blood and Fists.
So, if making a single 4e product in a line requires the rest of the existing line to be converted or abandoned, how is this not a "poison pill" albeit a smaller one aimed at product lines rather than the entire companies? And, I am asking this out of true curiousity rather than sarcasm or anything else.