It's not like those folks are free. We pay a lot of tax dollars to protect the Lithuania's And Montenegro's of the world. And in return we get some half willing but not particularly useful allies. The US lost over 2,000 soldiers killed in Afghanistan. Lithuania lost one. And it's not just a story of relatively poor or developing countries. Belgium lost one soldier. Portugal lost two soldiers. These were all countries that sent a token force and limited them to things like logistics and training the Afghan army, not infantry operations. They aren't the only NATO allies. I can't imagine a war where we protect Lithuania where the US would lose one soldier proportional to our population.
So while some of these countries might be willing members of a defense pact, they don't really consider it a mutual defense pact.