The implementation you use is your own choice, not a law of nature or part of either protocol. This is a classic responsibility-evasion cycle: Users: We can't

The implementation you use is your own choice, not a law of nature or part of either protocol. This is a classic responsibility-evasion cycle: Users: We can't get equipment that supports this (or supports it well, or makes it easy), so we won't deploy it. Vendors: Users aren't demanding it, so we won't support it. The result is a suboptimal local equilibrium and an obvious market failure, but you invariably get loudmouths on message boards saying "Look, the Market (Peace Be Upon It) has spoken!". Gets a bit old. The right answer in this case is probably coercive government regulation of the form "No IPv4 packet shall cross a property line", but I'm not holding my breath.