"for free" is not the best wording, no.
However, let's look at this another way. Say we have one person who is much wealthier/access to resources than a lot of other people, but who gets some limited benefit from cooperating with other people to get things they need. (exotic things that are rare in their locality/sphere of influence) They agree to build a fancy house along with the people they trade with.
The other people help, as best they can (or claim to be able to) in the construction and maintenance of that house, but the bulk of the labor and resources used to construct that house comes from the singular person with access to the greatest resources.
In a breakdown analysis, this person gets the LEAST utility from the partnership. They outlay the most material expense, and get significantly less benefit for that expense, than do the other partners in the agreement.
That is a better way of looking at NATO, and how it gets funded. NATO member nations get the benefit of the research dollars spent on arms development by the US, because the US shares that produced knowledge with NATO member states. The actual costs of producing that knowledge are not diminished significantly by the "fair" contributions of the other members.
To put it a better way, the relationship the US has with NATO is not symmetrical. It is doubtful that even combined, the other NATO members could repel an actual threat to US national security on a military theater, because their combined contributions to NATO are not a significant fraction of the US contribution, and if the threat can overpower the US, it will clearly overpower all the rest of NATO combined.
Which is a nice way of saying that NATO is not able to live up to the mutual protection nature of the agreement.
To live up to that agreement, the other NATO members must contribute substantially more to the maintenance and improvement of NATO forces and armorments.