how is it cheating when your just gaining knowledge about tactics etc... by reading articles
The same way it would be cheating in an exam? You can't read up on tactics because it gives you an unfair advantage! Anwyay it probably wouldn't help anyone stupid enough to do so :p
No, the purpose of an exam is to measure or test your knowledge. The main purpose of online games is to learn. Note this concept of testing versus learning.
If chess.com really wanted to use live chess as a measurement tool, rather than a learning tool, then that's up to them. But I believe the entire site is mainly devoted to learning. If you really wanted to test your skills, there are tons of tournaments out there. There are FIDE, USCF, CFC ratings and more. We don't need another online rating that's probably inaccurate anyways.
Would you be allowed to play an OTB game with a chess opening book beside you? This is exactly the same as reading opening theory from wikipedia while playing a chess game. It comes down to morals at the end of the day. If you really want to improve opening theory you should learn the theory in your own time, and then play a game. If you can't remember it midgame, then you know what to practise during your next training session. The fact that show many people think it is ok to cheat, be it mild or extreme, just goes to show the fewer morals that are enforced in todays upbringings. In the words of RJ Dio 'If you listen to fools, the mob rules'.
In an OTB game, the main purpose is to determine who is the better player, and award prizes based on that. Players pay money to enter tournaments. If you are a professional, your rating could determine your income. These games are absolutely serious. In short, these games are a test to measure your playing strength.
In an online game, the main purpose is to learn. The outcome of the game will have absolutely no impact on anything except our pride. You won't win $1000; you won't increase your income; you won't get a GM norm. In fact, if you are using a nickname, nobody knows who you are. So why play live chess if winning a game has so little impact? My answer: to learn. The exclusion of books/articles from live chess decreases its effectiveness as a tool for improvement.