I thought it's consequent to use Bitcoin address signing for this contract
Yes, but you have to use Bitcoin itself to sign it. No one agreed on the signature algo, so everyone else is doing it wrong and they need to switch to use Bitcoin's. If you would have signed it with bitcoin-qt or bitcoind, this wouldn't have been a problem.
Thanks for clarification.
If that's the case (everyone doing his own thing), it is not useful yet, since people not working with bitcoind (like /me) can't reliably sign / verify.