In the UK at least it is nothing to do with trendy ecological issues. I think it stems more from our radically different approach to service. In the UK the default position is do the minimum needed to keep the customer happy.

Which in my (admittedly brief) experience is the exact opposite to the default position taken in the US.

So I guess they think they are providing a better service by foisting straws on you.

An interesting counter argument though is bags at checkouts. In the UK until recently they have always showered you with endless bags whether you wanted them or not. So it was always a shock in the US to have to explicitly ask for a bag (when I needed it).