Because it will be argued in Congress, if I'm reading it right. They could rubber stamp the Bush program or add some small tweaks but I know Kennedy, Pelosi, etc. will do everything to get media time to bash Bush and show their moral superiority by expressing their concerns for jailed terrorists.
As for the law itself it may be an atrocity of legal reaonsing, some here think so.
What specifically?
The part denying jurisdiction is clear, concise, and directly authorized in the Constitution.