Yet it's not Riots fault if their breaking some of the borderline rules, yes there are some rules that don't generally pass the line, but Riot still perm bans it, which of course could be changed (i.e. warnings and scaling suspension like you stated), but some of the rules are already told from Riot in their Terms of Use and Tribunal (Which unfortunately is going under some maintenance at the moment), but some of the parts in Terms of Use state reasoning to permanent bans:
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"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."
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"2.2 ... You understand and agree that using the Riot Services comes with the risk that your account may be terminated or suspended and that, whenever you use the Riot Services, you’ll bear this risk in mind and always conduct yourself appropriately."
It is solely their decision on whether or not the ban should be proceeded with. If it's a yes and you're violated a rule then Riot has all rights to ban you, it's their game in fact. It can also be on the behalf of the community, i.e. Flamers, Trolls, Rude/Unsportsmanlike Behaviours. I see countless people blaming Riot even though half of it was viewed and token action upon our own community, the only thing Riot did was judge the report.
They also have "Code of Conduct" (Section 5) which explains all in-game behaviours that have risk of suspension. Generally people never read stuff like this, which will cost them these bans, this is in no way Riots fault.