While it is true that the "2% rule" was informal until recently, it is a fact that Europe has decreased its defense spending significantly over the years, from an average of 2% of GDP in the 90s to 1.5% in 2014 when the agreement to go back up to 2% by 2024 was made. In that time, US defense spending rose from 3.1 to 3.4% of GDP. NATO spending targets aren't "obligations" like you say, but don't be surprised when Americans don't want to support Europe's defense so that Europeans, who constantly hate on the US for being "world police", can afford to spend more on social programs. It's kind of sad that the only thing that got the major European countries to start taking their own defense seriously is a neighboring country being invaded.

They're agreements of all members.