How is it just that if a suspect can be questioned if they don't ask for a lawyer, but cannot be questioned if they do ask for one?
Note: ignore the first "if" in the title, that was an error.
This just means that they must pay for a lawyer to avoid questioning, unless they are indigent, correct? I don't know of any principle that would make it fair to question a person who demands not to be questioned, but unfair to question them if they instead demand a lawyer. Only that it makes it easier for police, but giving less protection to a person who is very thrifty with money, less educated, or less intelligent is not a question of justice.