again they have the full right to ban you because you do not buy anything in this game it clearly states that in 2.1/2.2 of their tos:
2.1. How can my account be suspended or terminated? (If you break the rules, Demacian Justice will be visited upon your account!)
Three different people/entities can suspend or terminate your account:
2.1.1. You. You may terminate or suspend your account at any time by contacting us at [email protected].
2.1.2. Us.We may terminate or suspend your account if we determine, in our sole discretion, that:
(a) you have violated any part of this Agreement;
(b) we have stopped offering the Riot Services in your region; or
(c) doing so would be in the best interests of our community, the Riot Services, or the rights of a third party.
We may make such determinations, among other ways, by using automated systems and machine learning tools.
2.1.3. The Tribunal®. The Tribunal® system allows our community of users to police the compliance of other users with the “Summoner’s Code,” which outlines the principles of ideal game play behavior, as well as with our other policies. Community members, including you, may submit descriptions of activity and actions of certain users within the Game, and collectively determine if that particular user was in compliance with our policies. If The Tribunal determines, in its sole discretion, that you have violated one of our policies, we may suspend or terminate your account on behalf of the community.
2.2. What happens if my account is terminated? (No LoL for you.)
If your account is terminated, you’ll no longer have access to it, including any of the associated data or content (e.g., champions, skins, Riot Points, etc.). You’ll not be entitled to any refunds and we’ll have no liability to you. We also reserve the right to terminate any other accounts you may have created, as well as your access to any other Riot Services (also without any refunds or liability to you).
And to get around you claiming you bought it and therefore have protection they added this in 4.3
4.3. Do I “own” the Virtual Goods I unlock? (No. What you “unlock” is not the virtual good itself, but rather, a qualified right to access it in the Game.)
You have no ownership or other property interest in any of the Virtual Goods you unlock, regardless of whether you acquired access to those Virtual Goods using Riot Points, Blue Essence or Hextech Crafting. Virtual Goods have no monetary value. You can’t redeem them for cash. You can’t obtain any refunds for purchasing Riot Points or Virtual Goods, except as expressly permitted by us. You can find our current content refund policy here.
We have the right to delete, alter, move, remove, or transfer any and all Game Content—including Virtual Goods—in whole or in part, at any time and for any reason or no reason, with or without notice to you, and with no liability of any kind to you. We don’t provide or guarantee, and expressly disclaim, any value, cash or otherwise, attributed to any data residing on servers we operate or control, including any Game Content attributed to your account. The sale or transfer of your right to access certain Virtual Goods, Blue Essence, Riot Points or Hextech Items may only be conducted via services approved of or provided by us, if any.
You don't own anything because they actually grant you a semi permanent lease for the money you spend. By banning you they stop the lease.
So yes, if they really had a mmr ban system it would just be way easier to ban you and completely legal.