Let's imagine for the sake of argument that any two players who are leveling to 30 normally will gain exactly the same amount of IP.
Player 1 spends all his IP buying champions because he wants to try more champions. Player 2 buys the champions Player 1 bought with RP and spends his IP on runes.
In game, when Player 1 and Player 2 end up playing against each other in mid lane, Player 2 has a material advantage of the bonus stats from their runes as compared to player 1, who will either have a smaller champion pool or less runes then the player who bought RP.
Buying RP doesn't get you runes, it allows you to buy champions so that ALL your IP can be spent on runes, when normally you'd want to at least get a few champions.
Just because it's not "if you pay for a subscription, you get all this cool stuff which non-playing players can't get" doesn't mean it's not pay to win.