In an analogous situation, if a shop mis-labels the price of an item (let's say, they forgot a zero or two for a $1000 item), the customer theoretically has the legal basis to purchase it at the labeled price. How is this scenario any different to a block chain contract with a bug?
Further, in many places your example itself falls apart.
https://smallbusiness.chron.com/company-advertising-price-wr... https://www.findlaw.com/smallbusiness/business-contracts-for...
There was a case like that in the early days of online shopping, which was actually decided in favor of the customer. The prices of items in the shopping cart were stored in form fields, and the customer edited them before placing the order. The court considered that a counter-offer which was accepted by the store. (It probably wouldn't go that way today; for one thing online stores have gotten smarter about trusting data from the client.)
That's a bit different from simply editing the client-side view and never informing the merchant about the change, of course.
There is no way that AT&T, Comcast or Bank of America will ever agree to a smart contract written by an ordinary customer. As always, the companies will write their own contracts, and customers will either accept them, or leave. And if the company has monopoly power, tough luck.