They certainly don't make it clear when you sign up.
Like you I mainly use an Amex card, but this is good for all those 'other' places.
One thing is that I seem to sometimes get an odd number of miles. How can you get an odd number of miles when it's 2 for each £ spent? I asked them that and they said they could give me an explanation of why but they didn't even understand it themselves. Strange.