Haven't we all heard this phrase commonly said at a project decision point? As with all rhetoric, it offers truths when properly applied and an intellectual shield that can promote disengagement and may even lead to negligence when misapplied.
Often a client or a boss pulls rank, and individuals who disagree with this direction mutter the aforementioned phrase, which ushers in an acquiescence absolving their investment in the argument. So they move on. Provided the decision is not unethical and the correct decision maker is indeed making it, then this is the right mindset to apply. In other contexts, this phrase can lead to individual and team disengagement with the loss of autonomy and, in the worst circumstance, eventual negligence.
One example is a software engineering team discussing priorities with the product owner. Several team members want to improve the CI/CD pipeline for quicker and more reliable deployments with a possible value of a 35% reduction in the time to deploy. Other team members want to enhance the sales funnel that is expected to bolster the bottom line of the application. After a discussion of the pros and cons of each suggestion, the product owner decides that the CI/CD pipeline can wait and that the improved sales funnel is more important. As the meeting draws to a close, several employees mutter this phrase and begrudgingly begin to work on the improved sales funnel. In this context, the product owner is doing their job appropriately by providing direction, and the team is doing their part to work towards that direction. The phrase "They're the ones signing the check" helps employees to distance themselves from the discussion and put their emotions behind them so they can get to work.
On the other hand, I've frequently heard this phrase uttered by individuals who have little to no investment in their work whatsoever. An individual pitches an idea but they're not willing to fight for it. To stick with the previous example, after showcasing the reason to improve the sales funnel, an employee suggests improving the CI/CD pipeline instead. Initially, the team doesn't understand the reason and advocates for the improvement to the bottom line as being more important. The employee mutters this phrase and drops their case for the CI/CD pipeline. Unbeknownst to the rest of the group, let's say there was a security breach and several of the passwords to the cloud cluster were leaked, but the employee did not mention that to anyone since they faced the initial resistance. Two weeks later, the development team finishes their sales funnel work, deploys it, and discovers that their cloud cluster has been hijacked for cryptocurrency mining! Since the employee who proposed prioritizing the CI/CD work did not bring the breach up to the others, they are guilty of negligence and could face a lawsuit from the company.
Now, this example is most definitely contrived to demonstrate extreme consequences (I do not know of anyone who would go this far), but there are times I have seen individuals mutter this phrase after their suggestion gets shot down even though they offered little support for it. Rarely in these cases will the reasoning behind the proposal be self-evident. It is the responsibility of the employee, as laid out by the burden of proof principle, to present their reasons for their recommendation. However, guarded by the adage "They're the ones signing the check," the employee has distanced themselves (and their concerns) from the discussion and ultimately allowed the product to suffer.
There is a good time for this cynical truth to be uttered and for others to recognize this truth, that yes, the client is signing the check, but this complacency needs to be monitored so that it does not become an excuse for intellectually checking out and beginning to alienate yourself from your work.