Wall riding is different as it is actually against the rules in the game, there are just ways of doing it that avoid detection by the game which is an exploit and illegal
Here we go again, How can on the inside radius of a corner all 4 wheels being off the actual normal asphalt track racing surface not be considered an "illegal" exploit that is plainly shortening the length of a laps full distance while rubbing the outside track barrier while remaining fully on the tracks allowable asphalt surface is considered 'illegal'?
Certain corners the limits do not allow such generous allowances and you will be penalized for the much less of same amount of the car leaving the defined racing surface inside the outer white lines of the track.
The same as some walls will give you a penalty for contact some will not.
Both situations are dependent on location and proper implementation of correctly pushing the limits without the game issuing a penalty.
Again both are actually just pushing the actual limits of the rules and limitations of what the game will allow before it recognizes the action as illegal and an act that calls for the racer to be penalized for the action.
Both actions are available to be "exploited by all racers" during the entire distance of the contest or race.
Unlike the pit glitch which can only be exploited by one driver at the beginning of a race and therefore is plainly a cheat as they are gaining an advantage over the other racers that they do not have the opportunity to gain that same advantage within the contest.
Corner cutting is just as much against the rules of the game as wall riding and there are just corners that the limits allow that to be exploited just like there are walls that allow contact without penalty.
Your saying one is cheating and one is not is just you being okay with one exploit that you may feel like it is okay and may even exploit yourself and saying the one you do not like as cheating.
Take off the blinders and admit that both situations are location and game allowance dependent, can be used by all racers throughout the contest and other than what your preferences may be personally for the rules these are the same and you cannot be cheating just exploiting one or the other either they are both cheats or neither are cheats, you cannot have it both ways.
Personally I do not agree with either being allowed in the game, inside radius corner cutting should pull a penalty if any part of the car actually exceeds the outside edge of the actual curbing and contact with the wall should at minimum cause a slowdown of a cars pace which would negate any brushing against or trying to use the wall as a corner guide.