The United States is not under any form of responsibility to other states to provide them with military and other aid. Aid has always been provided with strings attached to those whose receipt of aid is considered beneficial. It is within the purview of the US to decide who they give aid to, based on their own priorities. This is simply part of the diplomatic process. If the US wishes to provide aid to countries that sign up to treaties then this is its right, it is perfectly normal to provide a sweetener to encourage states to sign up while punishing those that don’t.  This then is a good reason why these European states should not pull out of their BIAs. To do so would mean losing the financial benefits being provided. At the same time it would also show that these states are not to be trusted when they sign up to international agreements.