Usually, "money" is defined as something that is generally accepted as payment for goods and services (and for the settlement of tax obligations).

In practice, "generally accepted" in interpreted with some lenience. So it doen't mean universally accepted or something. But, for instance, if there's a chip on your employee ID on which you can store money to use for nothing but the firm's cafeterias and other in-house shops, that's not considered money.

I think the lawyer's argument that M&M miles are broadly accepted has little merit. I see LH's reaction as a precautionary measure. Even if their chance of losing is fairly small, there's so much money at stake that they do it.