Sure, we CAN publish outside the GSL, but it just doesn't feel RIGHT to do so. In dmccoy1693's words - it feels like I'm not playing nice with WotC by doing that.
Given the generosity of the OGL, I agree it maybe feels a bit 'off' to use it to publish 4e product, that seems arguably a bit unfair, even though legal. Mind you AIR Kenzer published their 4e-compatible Kalamar Guide without either the OGL or GSL, and that doesn't bother me, as long as they stay within copyright law.