"I.14 C Stadium – Accommodation of visiting supporters
At least five per cent of the certified total stadium capacity must be made available for accommodating visiting supporters in a separate area.
This provision is subject to decisions by the competent bodies of the licensor and/or the local authority regarding safety and security (high-risk matches, etc.)." - Regulations: Club Licensing, FIFA. pg. 26

If this is a FIFA regulation for club licensing is Seattle obligated to open over 1,500 seats to away supporters, not just 150. Portland and Whitecaps would have to open 5% of their stadium since they are FIFA licensed clubs. Why does the MLS seem to easily fall out of line with FIFA, and as the CBA negotiations went, FIFA seems unwilling to get involved in enforcing it's rules in MLS.