Why do you think that Sound Transit isn't required to use that depreciation schedule when the law directly says that Regional Transit Authorities (of which Sound Transit is the only one) are required to use that depreciation schedule?
Also, Sound Transit didn't write the legislation. They don't have that kind of clout in Olympia. If they did, explain to me how it is that they keep losing to WSDOT and UW, two state agencies that do have clout in Olympia. That UW Link Station is located where it wound up is prima facie evidence that ST has limited "pull" with the state.
(Notice how amendments were brought to modify the depreciation schedule at least twice and were voted down in the legislature. Senator O'Ban, the vocal critic against Sound Transit, voted in favor of the bill containing the Sound Transit 3 authority--TWICE--before he decided he didn't like it.)
They aren't "required" to use that schedule, it sounds like they are the only ones using it, and Sound Transit wrote the legislation.
So I really don't understand your point. Sure we need transit, but the ends don't justify the means.