[CQ-Contest] Legalities and ethics of international remote operation?

[Barry]

On Saturday afternoon, during the 10m contest, I was surprised to be 
called by a very loud YO4.  Condx were such that the band was not open 
to central/eastern EU here in Colorado at that time.  Given the signal 
strength, I presume the YO4 was using a remote on the east coast of the US.

What are the legalities of such an operation?  Should he be signing 
W/YO4xxx?  Off the top of my head, I don't know if YO is a part of CEPT, 
but if not, what kind of licensing permission, if any, is required for 
such an operation?

Where does this type of operation fall in terms of contest rules and ethics?

Barry W2UP