They are arguing that the MOU definitively establishes the SODO site, which would violate the state's laws regarding environmental review, in which multiple sites must be reviewed.
Other sites ARE being reviewed, but they are contending that given the MOU pertaining to an arena in SODO that's it essentially all for show and not in good faith. The MOU, of course, is not final approval of the arena, which comes after the EIS, but the ILWU claims that Hansen has already chosen the site and its essentially final. It's really just a messy, illogical argument that rests on the fact that a private investor is willing to spend a pile of money at one site but not others, which is their right, and they contend that fact means the EIS isn't valid. It's nonsense really. It's hard to even explain clearly because it's such drivel. Of course, they're just making noise, unless they really believe the logical conclusion of their argument, which is that any private investor looking to partner with the public would basically have to put up money and just say "you guys pick the best location" in every instance.
Some of the whining has been along the lines of "What gives Hansen the right to choose the site?" etc., well his pile of money that he's free to offer for one site and not for others is one thing. They've also even suggested that sites in Bellevue should be reviewed, in an EIS that pertains to bonding by the city of Seattle.