No it's not.
The phone company charges the roaming phone because otherwise the calling phone would be charged at the roaming rate. If I'm in Hong Kong my UK clients may not know, and if one of them decides to call me for advice I don't want them paying for a UK to Hong Kong call to a mobile. They certainly don't want the charge either, so what happens is that they are charged at the local UK rate, as if they were calling to a UK mobile, and I pay the difference - the UK to Hong Kong charge, less whatever the call would have cost, were I in the UK.
It's a good system, not a rip-off at all.