The problem is what you mention, "scale". They do not have the scale of population or economy to protect themselves from their most likely predator (I hate to say it on this subreddit, but the proof is in the pudding that non-aligned nations generally do not fare well, I will say no more). They agree to contribute their forces in defense of other nations, they will receive friends to defend theirs. Are you saying mutual alliances should not exist?

The alternative scenario is that you have Poland, for example, feel pressured to take nuclear weapons to be able to pose a threat to their certain neighbor, and thats worse for everyone. We have seen this with North Korea: as they grew increasingly abandoned by China and USSR/Russia, they have grown more belligerent and focused less on conventional parity/deterrance and more on NBC warfare.

Also, as a veteran of the US military, anyone who joins is broadly aware there is the NATO agreement and thus there is implicit consent to the idea that one could be called to defend an allied nation. It is no different from Russian soldiers being aware they could be sent to defend Syria as per defensive alliance between the two nations, a nation that arguably shares few ideologies with Russia. You would find very few in the military opposed to the concept of defending friendly cultures. As for the "taxpayer" argument, is there an honest difference in having troops at home vs abroad considering most of our partners subsidize the costs of our bases? It is a arguably better use of taxpayer money to have troops "forward deployed" in military centers of gravity across the globe than to have them sitting in Georgia or Washington. Plus there are the hundreds of unspoken benefits, eg constant cross training with other militaries benefits everyone involved.