Under that line of logic, no one would ever cheat on their taxes because the IRS might investigate and prosecute you. However, cheating on taxes happens all the time. This has to do with the risk reward assessment. The risk is, the IRS will audit you, and make you pay back the back taxes that you owe along with fines and possible jail time. The issue becomes, that the IRS has such little man power that the chances of you being audited are very slim.
The same logic applies here. There is a chance that your game will be randomly screened and possibly punished by Riot. The problem is the massive amount of manpower that would be required to make this effective. You would need to almost double the size of the company, to hire people that only watch games for trolls, flamers, feeders etc. From a business prospective, that make no financial sense. Not to mention that the odds of you actually being monitored are tiny, there is no disincentive for people to trolling etc.