Theft is defined as "the action or crime of stealing," and stealing is defined as "taking (another person's property) without permission or legal right and without intending to return it." When a player purchases RP, they give money to Riot and Riot increases a number in a database that Riot owns and operates. When that player spends the RP, Riot decreases the aforementioned number and simultaneously makes another change to the account which that player is allowed to access. This change might be a flag indicating that a certain skin can now be selected, or an increase in the number of rune pages customizable from that account. At no point in time does Riot sell, rent, or license any kind of ownership of any of Riot's assets to any player. Players are simply allowed to access the account they create, with their authorized credentials.
This is no different from accessing any other private entity's services. If you notice that a wall at a friend's house is very plain and you buy them a painting to spruce it up, that painting now belongs to your friend. If your friend forbids you from coming into their house, which could be for no reason, any reason, or very good reason (e.g. that you continually broke clearly-defined rules and ignored all reprimands and requests to change your behavior), you don't get to keep the painting. You also aren't owed the financial value of the painting, nor does helping with the decor grant you any right to keep hanging out at your (former) friend's house.
