AA:

there may be differences in our social and legal cultures: we are fortunate to have Lionel to bridge the awesome gaps

In the United States, there would be little doubt of the social condemnation of FT: its actions, though enabled by carelessness, would be regarded as for more reprehensible than that carelessness; which would in any event be sufficiently punished without the depredations of a scamster like FT. [There is an old American case in which an unsuccessful defense was asserted to a charge of theft of mislaid property: that the defendant was a member of a sub-culture which saw nothing wrong in appropriating stuff lying loose. I don't remember what the sub-culture was--- perhaps recent English immigrants?]

In our legal culture, although FT has maybe hit on an innovative FORM of misbehavior, our bench and bar, finding no word in the dictionary through which to restore the socially-condemned misappropriations and deter their future imitation, would make one up. Would they be abashed at FT's indignant claim that its innocent behavior had been condemned retroactively and lawlessly? It would rather be suggested that FT, being as it is, is entitled to take advantage neither of ET's ineptitude nor of inadequate generalization in the prior legal names for misbehavior.

Not that there is lacking a sufficient doctrinal and verbal basis in existing US law for getting at FT. I started by supposing that there might be a more conventional basis than restitution, and I hope that someone will come up with a satisfactory tort name. However, the Hedley MS concludes with the always useful thought that CONTRACT might resolve a puzzling case:

IN US law, the buyer of the card could be regarded as purchasing options--- offers by ET to provide phone service at stated rates. The phone call could be the acceptance. In US law, a person who receives an offer which is clearly grounded on a mistake of fact [rather than of judgment] cannot conclude a contract by accepting the offer: 'snapping up' is the American legal pejorative for such a purported acceptance. That FT were aware of the mistake is evident from their behavior.