Well, the Constitution does allow for suspension of habeous corpus during invasion, and we've been invaded. But the section allowing that is in the part that describes legislative powers, not executive. I do agree that tribunals are legal - but they carry a tremendous potential danger to all of us, and should be treated as such.
We were attacked, not invaded. The feds have taken full advantage of the attack to curtail as much freedom as they can. The tribunals are a bad idea from several different points of view.
As you point out, they are being implemented by decree, not by legislation.
Bush & Co. get to decide who the terrorists are (much potential for abuse there).
The precedent is a long term blow to freedom throughout our nation. The potential for abuse from a left wing dictatorial type is tremendous.
The thing to keep in perspective is that we have not been 'invaded' any more than Pearl Harbor was an 'invasion'. That renders any Constitutional support for Bush's decision shaky at best.