I think that the new government in Poland realizes that George W. Bush does not stand up for his allies, and his successor will be unlikely to stand up for George W. Bush’s allies, so they are special military treaty ahead of negotiations on a US missile shield in Europe, such as the ones with Turkey and Italy.
For Poland, “The worst scenario is a situation in which Poland would agree to the shield, will incur the political costs and then the base is not built, because of a change of government in the United States”.
They are afraid of being left holding the bag.
With a year left in the reign of the Avignon President, it’s a sensible concern.