The Wikipedia article on the Sokol space suit seems to suggest that all personel traveling on a Soyuz vehicle must wear a Russian made spacesuit:
Each Soyuz crew member is provided with a made-to-measure suit for flight although, from the numbers made, it appears that the suits provided for ground training are re-used.
The suit is worn during launch and re-entry of the Soyuz spacecraft—the gloves are attached and the visor is sealed at these times.
Peggy Whitson suited up in pressurized Sokol-KV2 suit.
From this, it appears that American astronauts travelling on the vehicle would also wear the Soviet suit. Is this true? If so, why aren't the American suits adequate?