Because one can break gentlemen agreement against people with lower social status. Also, because low key bullying or lying about or whatever of people with lower social status is acceptable for gentlemen.
Again, you say that like it's a fact. Gentlemens agreements can be broken with everyone. Low key bullying or lying about is not acceptable for anyone for gentlemen. You seem to have a misunderstanding what "gentlemen agreement" means. It's not limited to upper class men, or men at all. Everyone can have gentlemens agreements with anyone, it means "two or more people who implicitly agree on a thing".