Decoding a BIP-70 Payment Request
We can also analyze transactions to make sure an adequate bitcoin miner fee is included. If the fee isn’t sufficient to allow the transaction to confirm on the bitcoin network on time, BitPay can return a helpful message back to the wallet to let the user know. Mistaken payments will never reach the Bitcoin network.
The wording suggests that a wallet sends the transaction to BitPay for approval and they forward it, but afaik that’s not what the BIP-70 does. According to the specification wallets broadcast the transaction via the P2P network like any other transaction. Of course BitPay can always choose to not honor a transaction they receive, but I don’t see how BIP-70 changes that.