Two counters to this. First, look at the comments. If what you were saying was a "thing you should know" or "common sense", I'd be the only person here saying otherwise.
Second, this is a video game. The rules are enforced by the game's logic. People don't sneak into the enemy base to kill the Nexus without killing any towers. Why not? Because it's dishonorable or indecent? No, because the game logic prevents you from doing so. If you're playing football and you get tackled, you can get right back up and keep running with the ball. The game's rules disallow it, but nothing physically prevents you from doing it. The written rules have to be interpreted and enforced by someone with the authority to do so.
That is the advantage that video games have over RL games. There is no human interpretation involved. What the game logic allows is allowed; what the game logic doesn't allow is forbidden. In Season 4 when teams started playing by sending 4 to a lane and pushing down the first two towers uncontested in the first few minutes, fans complained but no one said that the teams should stop doing that or that they were using a cheap strategy because they weren't good enough to win "the right way". Instead we wanted Riot to change the game logic to prevent or at least disincentivize that tactic. We want the teams to do as well as they possibly can within the parameters of the game, and we want the parameters to be set up in such a way that the result is an interesting game to play and watch.
When you 1v1, you're deviating from what the game logic explicitly allows. In addition to the game's rules, you're establishing a few more: 1 person to a team instead of 5, win by killing the other player/tower instead of the nexus. It is up to each player to clarify what extra rules are in place, because the two of you are the only ones who are going to enforce them. And in doing so, they MUST be clear. Sports rules leave little to be subjectively interpreted. In basketball, the rulebook doesn't say "You can't go for too long without shooting"; it establishes a 24-second shot clock. It also clearly defines what a "shot" is (technically a "field goal"). The rule you're suggesting, even if it were stated beforehand, is too vague to be objectively enforced. Can everyone agree on what normal summoner spells are, or what a normal build is? You cannot enforce it, and you didn't even say it, therefore there is no problem with Fizz breaking it.
