One-way flight on round-trip ticket?

I'm thinking of flying a route in Europe that doesn't seem
to be covered by any of the budget airlines. I found a
reasonable rate on one of the major carriers -- but the
"reasonable rate" is for a round-trip ticket, the one-way
fare is outrageous. And as you've guessed, I'm planning to
fly only one way.

Here in the US, I know people who regularly buy round-trip
tickets in these situations and discard the return ticket.
It's my understanding that the airlines all have policies
that specifically prohibit this, but that, practically
speaking, there's nothing that they can do about it.
Specifically, I've read that even if they decided to charge
the difference to your credit card, you could easily (and
successfully) contest the charge, as their claim that they
have authorization to asses that charge to your card has
dubious legal force.

Does anyone know if the situation in Europe is similar to
that in the US? In other words: How risky (financially
speaking) would it be for me to buy a round-trip ticket and
discard the return portion?

Thanks for your help!