On the face of it, it definitely seems like an odd question. Why would you ever reject a job based on it paying too well.
However, while some people may feel comfortable to fight for as much above their minimum as possible - there are also many people who have a comfortable salary range where they feel their skills would meet the expectation set by the salary.
For example, there are people who feel comfortable delivering good value on a £30K salary. Given the potential to make £50K, they may still see this as a good jump and within their reach. However, if you offered them £200K, they will steer away from the position - as they feel their skills cannot match the offer, and that they will be out of their depth.
If your maximum is genuinely "uncapped", then by all means do just tell the recruiter so (and if they persist, find a new recruiter).
But there are definitely people who would take real persuasion to look at an opportunity that "overpays" compared to what they think they are worth. In those cases, it would be a waste of the recruiter's time to apply for them - as the person does not truly feel comfortable taking such a role (even if this seems irrational to somebody who fits the "pay me as much as possible" category).
Of course, the most common way for a recruiter to handle this entire thing - is just to ask "what's your expected salary range".