Why is it completely acceptable to feed in ranked,
It's not. The problem is likely in your definition of feeding. People can have bad games where they're camped or simply aren't doing so well -- that doesn't equate to intentionally feeding.
or to buy an account then play in it in ranked,
It's not, and is against the ToU (III.D)
D. Account Sales. The Account supplied to you is personal to you, and Riot Games does not recognize and expressly forbids the transfer of user Accounts. You shall not purchase, sell, gift or trade any Account, or make any such offer, and any attempt shall be null and void. Any distribution by you of your Account and/or your Login Credentials (except as expressly provided herein or otherwise explicitly approved of by Riot Games) may result in suspension or termination of your Account.
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or to just mess around all game
It's a video game. It is meant to be played for fun. There's a large number of people who are content where they are in ranked and play <gasp!> for fun! Why they're playing in ranked rather than normals? More of a challenge. Just because they're "messing around" (a seriously nebulous phrase that encompases everything) doesn't mean they aren't also trying to win.
or trollpick champions you dont know how to play (I.E. Teemo adc.)
Same as above, they aren't doing these things to spite you or to lower your chances of winning (unless they say as much, in chat). The meta is not enforced and they're allowed to try different strategies to their heart's content.
to play ranked at all without any knowledge of how to play to win
By all means, define a basis for requisite knowledge of how to play to win. I'm pretty sure everybody that's level 30 (that didn't buy an account) has that knowledge. Application of it may vary.
To me; you sound like you're frustrated about recent losses that you feel were the result of other people (beyond your control, et al). I'd recommend taking a short break and reflecting on why you care about winning as much as you do. Sure, it's a competitive game; but at the end of the day, it has absolutely no bearing on your life. Win or lose, you've still wasted that hour. There is no substantive benefit to playing this game for anybody other than the pros.
I'm not going to whiteknight and say trolls deserve the right to troll, that's dumb. My argument is that you're defining everything you disagree with or that is possibly negative to be trolling, which is an incorrect definition. Trolling is attempting to derive enjoyment from the suffering of others. Playing Teemo ADC isn't trolling unless the intent was to make you squirm, curse, and pitch a fit. Doing poorly in lane isn't trolling -- trust me, they're way more frustrated than you and your complaints don't help it any.
Just take a breath and remember that this is water.