No Sounders could have played in the game anyway -- since the All-Star game conflicts with Seattle's CONCACACAF Champions League opener on July 28 -- however, Sounders were as eligible as anyone for the honor of being named to the team.
By my quick count, Seattle is one of only three league teams not represented, along with San Jose and Kansas City.
Here's part of the release from the league:
MLS Commissioner Don Garber and MLS All-Star Team head coach Bruce Arena of the LA Galaxy have made their selections to complete the 23-man 2010 AT&T MLS All-Star roster. The MLS All-Stars will take on Manchester United in the 2010 AT&T MLS All-Star Game on July 28, 2010 at Reliant Stadium in Houston, Texas (5:30 p.m. PT, ESPN2, TeleFutura). The 2010 Commissioner’s selections are Houston Dynamo star forward Brian Ching and “MLS Original” Jaime Moreno of D.C. United. Due to international club competition for five members of the All-Star First XI and an injury to midfielder Kyle Beckerman, Arena selected 11 additional players for the game day roster: Goalkeeper Nick Rimando (Real Salt Lake); defenders Kevin Alston (New England Revolution), Jonathan Bornstein (Chivas USA), Wilman Conde (Chicago Fire) and Heath Pearce (FC Dallas); midfielders Sebastien Le Toux (Philadelphia Union), David Ferreira (FC Dallas), Brad Davis (Houston Dynamo), Shalrie Joseph (New England Revolution) and Jeff Larentowicz (Colorado Rapids); as well as forward Juan Pablo Angel (New York Red Bulls). Arena and his staff will select the starters based on player availability and tactical considerations. Under the CBA, a total of 32 players will be named All-Stars. After the 23-man game day roster, the remaining nine players receiving that title will come from the weighted voting, but will be inactive for the All-Star Game. The inactive, or alternate All-Star roster, will be announced at a later date. 2010 AT&T MLS All-Star Game Roster: Goalkeepers: Donovan Ricketts (LA Galaxy)*, Nick Rimando (Real Salt Lake) & Defenders: Omar Gonzalez (LA Galaxy)*, Chad Marshall (Columbus Crew) *, Jamison Olave (Real Salt Lake)*, Jonathan Bornstein (Chivas USA) &, Heath Pearce (FC Dallas) &, Wilman Conde (Chicago Fire) &, Kevin Alston (New England Revolution) &. Midfielders: Dwayne De Rosario (Toronto FC) *, Marco Pappa (Chicago Fire)* Javier Morales (Real Salt Lake) *, Guillermo Barros Schelotto (Columbus Crew) *, Sebastien Le Toux (Philadelphia Union) &, David Ferreira (FC Dallas) &, Brad Davis (Houston Dynamo) &, Shalrie Joseph (New England Revolution) &, Jeff Larentowicz (Colorado Rapids) &. Forwards: Edson Buddle (LA Galaxy) *, Landon Donovan (LA Galaxy) *, Juan Pablo Angel (New York Red Bulls) &, Brian Ching (Houston Dynamo) +, Jaime Moreno (D.C. United) + Unavailable: Kyle Beckerman (Real Salt Lake) * Denotes selection to First XI Team+ Denotes a Commissioner’s Selection & Denotes Coach’s Selection Online fan balloting comprising 25 percent of the combined First XI vote, with MLS coaches and GMs (25 percent), media (25 percent), and players (25 percent) accounting for the remainder. Players and coaches/general managers could not register votes for players on their own teams.
More information at the MLS website.