A real life example might be that when a flight attendant asks this type of person whether they want pretzels or crackers, they ask if they can have both. If they can't, it's no problem. They just like to probe and see what they can get if they want more than what seems to be available to them. There's an audaciousness about them in that many people wouldn't do that, but this person still has morals and doesn't necessarily lie or steal to get what they want.
These are my opinions, but opportunism ("opportunistic") implies a lack of morals and evokes thoughts of graft. Self-indulgence ("self-indulgent") implies excess, and maybe gluttony. Immoderation ("immoderate") has a similarly "sinful" tone.
Taking another path, the best antonymic phrase I can think of for this concept is abnegation, but it also seems to have a moral connotation (this time positive) of sacrifice and self-denial. An example sentence might say that "one who abnegates denies themselves of things they could otherwise possess, whereas a ____ person seeks to get things they may not have otherwise had if they didn't probe for them."
Is there an adjective for this type of person with a more neutral "feel" and definition? I have tried looking at definitions and synonyms of the words mentioned above (and others) in Merriam-Webster and through Google.