Two issues:

1) Tarrif rules

Usually the tariff sets out that the first sector is fixed once you are within the ticketing deadline. So if the passenger doesn't use it, the passenger may be subject to cancellation penalties.

2) Fare calculation

RTW fares vary according to the point of origin. If the first segment is international and skipped then the fare calculation goes out the window, because the actual journey starts in a different country.

It is cheaper to start a RTW in Canada than the US. If you just dumped (say) a YVR SEA as the first sector, then properly the airfare would have to rise to the price for a US point of origin.