Most of these rules have been around on that subreddit in some form or another for quite a while, so I think any direct reason is out of date. The best answers I can give you are
Hacked accounts and selling/buying accounts - All range from legal issues (in the case of identity theft or fraud) to ToS/EULA violations, which has been against the rules for as long as I can remember. ToS/EULA rules are there for a variety of reasons, but also largely to prevent posting how to exploit.
Perm/Temp ban threads - These were something no one really wanted around. After Riot implemented chat restrictions (and before that too, really) the subreddit was flooded with low quality posts from people who claimed, "I know I've been toxic sometimes...but really I didn't deserve this ban just because I made a racist remark about someone and told him to kill themself!!!!!" Those threads then led to people constantly spamming to "summon Riot Lyte (Riot's head of behavioral division/unit/whatever they call it)" to post chat logs "smiting" them for their behavior by posting proof in the form of chat logs. Obviously trolls flocked to this and started posting toxic accounts just to get attention. Personally, I feel that the mods have been pretty fair with this rule, often letting certain threads slide if they provided enough proof that it was no longer about being banned, and more about whether the system is flawed or not.
RP crediting issues - For anyone that doesn't know, RP is League's real money currency. I don't really know about this rule, other than from my perspective it fits in line with these other rules, and reddit is probably the worst place to be trying to discuss payment issues with Riot Support. Too little that can be done in a public forum where posting personal information is not allowed, and just trying to "get a post to the front page for visibility" just lowers the quality of the subreddit.
Stuff related to Riot Support - Probably the only remotely questionable rule, but the intention was (presumably) largely to combat the huge number of low quality posts that reddit couldn't really do anything about, because they were account support issues that only Riot had answers for. Mods didn't want people trying to "bait out" Riot responses. Not sure if it's really relevant anyway, since looking through the current subreddit rules the only variation of this rule I can find that's active right now is directly disallowing only tech support issues, and even then the only time it's really enforced is with personal tech issues. Tons of threads that are game-side problems (low FPS on certain patches, sound issues with new skins, etc) hit the front page all the time.