Re: le -sigh-
"Presumably the US has no such equivalent, in theory or in practice?"
No. In fact, if a company looks at customers first without considering the investors, the investors can SUE the company for failure of fiduciary duty and be compensated for it. Remember, the investors are the actual owners of the company (it's their money on the line), so property rights kick in, and doing something against an owner's wishes is considered defrauding that owner.
"Obviously nobody ever bothers with this, and the consequences of ignoring it are nil, but this is The Law as it has been in the UK for a few years."
Because it's very, VERY hard to tell an owner what to do with the stuff he/she owns. They can legally argue that laws that limit an owner are a restraint on their property rights and thus an unfair limit on their freedoms. The only reason the UK can get away with it is because, due to the precedent of the superiority of the Crown, property rights aren't as strong, but in just about any country, no owner is forced by law to provide service of any kind. Push come to shove, they can pack up and leave taking everything with them. Economics pretty much foretells this fate if the Supply and Demand curves stop intersecting.