TB Mastery Swap

Ok so I went into TB and I didn't select anyone yet. I then went to a friends profile to look at their Masteries. I went back to the TB then selected my champion. To my surprise I then had the persons Masteries on my champ. I tested this 3 times then went into a game. Their Masteries was working for my champion while I was in game. People say this is a known bug. If it's known why not fix it? Ik a lot of people are going to be mad at me for tell you guess this but to bad. Just helping out. Thanks have a wonderful Day.