I think there is a concern that the goverment may decide "When we debated the bill, and passed the bill, it was not a tax. However, at this point in time, we deem it expedient to declare that it is a tax. Let it be so."
I don't think that would work, since the "we" mentioned above was the Legislative branch, but the case is being defended by the Executive branch. The Executive can't change the intent of the Legislative.