Well, there you go, Walther's Warranty only covers the original buyer, so it really wasn't eligible for repair/replacement under warranty, and they don't sell frames separately either, so you can't fault Walther for their refusal to do so.
Furthermore, very few firearms manufacturers would have repaired/replaced the frame under the same circumstances, so isn't Walther in particular refusing to provide an otherwise common service.
Smith & Wesson is somewhat unique in the way that they'll often deviate from the terms of their Warranty by coving used firearms, but then again, they're not really consistent as they'll typically refuse if they're expressly informed that the firearm was purchased used, so it's really more that they neglect to ask whether or not someone who contacts them regarding warranty is the original owner of the firearm.
In fact, this is why I generally avoid purchasing used firearms unless they're extremely cheap or otherwise long out of production anyway, because you really cannot be entirely sure what the original owner subjected it to, ergo your Walther PPQ's frame may have cracked because the original owner (Let's call him Bubba...) bought a "Bag O Bullets" online filled with mixed +P+ loads and thusly overstressed the frame with ammo it wasn't rated for causing premature failure of the frame.
