This isn't a weakness at all. Your wallet knows what it can spend, and you only consider payments paid when they are paid according to your terms.
A wallet isn't tasked with knowing about crazy things that it might be able to spend if you modified the code— what you're saying is completely isomorphic to saying that the US Dollar has a weakness because someone could put a $100 bill in a locked safe with your birthday as the combination and bury it in your back yard without your knoweldge, and you wouldn't know that you could spend it.
Any contract use with Bitcoin requires you to have software that understands the contracts— no wallet does or will ever show arbitrarily encoded contracts written by a third party. Otherwise, someone could start "paying" you using 1 of 2 payment with one of them being a private key your wallet knows— then claw the funds back after you thought the transaction was settled. Because a contract could do almost anything it would never be safe to just automatically accept them, in any system, and so nothing does or will.
A wallet isn't tasked with knowing about crazy things that it might be able to spend if you modified the code— what you're saying is completely isomorphic to saying that the US Dollar has a weakness because someone could put a $100 bill in a locked safe with your birthday as the combination and bury it in your back yard without your knoweldge, and you wouldn't know that you could spend it.
Any contract use with Bitcoin requires you to have software that understands the contracts— no wallet does or will ever show arbitrarily encoded contracts written by a third party. Otherwise, someone could start "paying" you using 1 of 2 payment with one of them being a private key your wallet knows— then claw the funds back after you thought the transaction was settled. Because a contract could do almost anything it would never be safe to just automatically accept them, in any system, and so nothing does or will.