Interesting that Walker came to the USHL after playing in a pro league (Czech).
True, and many former CHL players have played in the USHL. The keyword being "former". As long as they're not under a current contract and playing without pay, their pro status doesn't affect the amateur status of the others in the league. In theory, a former NHL'er could play in the USHL, but it's difficult to conceive of a scenario where a player goes from having an NHL contract to having no pro contract whatsoever.
This is similar to the reasoning for the mixing of pros and amateurs in unpaid tournaments (like the WJC) as well, except in those circumstances, the players CAN be under contract in a different league. Team USA would have a difficult time competing in the WJC with only pros or with only amateurs, so I think it's fair to make exceptions when playing for your country. The main thing is that nobody is getting paid to play for their country. That would be a deal-breaker for the NCAA.