Applying the wrong mulligan at FNM Draft
That's an interesting one, Frédéric.
This is comparable to players going “hey, what if we start with 8 cards each?”, “lets try something different, lets play two lands a turn each” or “want to play with our commander decks?”. These are all variants of magic, but they are not the specific variant of magic that everyone in the tournament signed up to play. We want to keep things consistent, while it seems Bob's offer was fair, it can cause problems later down the line. Imagine Alice is paired against Bob next round, and, being a new player assumes that the London mulligan is normal, or that Bob is ok with all his opponents doing it. Bob keeps, Alice mulligans and performs a London. Bob, realizing what she did, says that he would have taken a mulligan if he knew he could have London'd. What if Alice gets paired up against someone else, and thinking that it's normal, London mulligans? What if word spreads to the games next to Bob and Charlotte, and they also agree to London mulligan, then half the room is essentially playing a different format.
If it happened again, I would explain that I would like the players to use regular mulligans to keep things consistent, giving one of the examples above.
That's my two cents, anyway. Thanks for the question!