If the wallet software is doing automatic CoinJoin (for example), then typically one or several of the other participants will broadcast the transaction as soon as it is complete.

If the spec said that wallets must not broadcast until they receive a PaymentACK (if a payment_url is specified), then you'd have to violate the spec to do CoinJoin.

And even if you don't care about CoinJoin, not broadcasting the transaction as soon as the inputs are signed adds implementation complexity (should you retry if payment_url is unavailable? how many times? if you eventually unlock the probably-not-quite-spent-yet inputs, should you double-spend them to yourself just in case the merchant eventually gets around to broadcasting the transaction, or should you just unlock them and squirrel away the failed Payment so if the merchant does eventually broadcast you have a record of why the coins were spent).