Right genuine question here because I feel like I am missing something. Full disclosure first. I am a Yank. So there may be bias involved but I have tried to look at this as objectively as possible.
Here is my issue with the situation. If this happens elsewhere on the pitch with two outfield players it is a foul. Player A wins the ball and is caught by Player B. That is a foul elsewhere on the pitch. Does the fact it is a keeper change this for people? Yes Mendy dives with hands. But imagine he was stretching with his foot to reach the ball and then gets caught. Is that different?
Second point. The video of the incident that was posted on this sub is not a very good one. There was a video posted on the Chelsea sub: https://youtu.be/SvZmRv6U_s0
This angle shows more of a follow through. If Bowen wanted to avoid Mendy, he could have. Now you could argue that he has every right to go for that ball, especially in the dying embers of the game looking for an equaliser. And Broja does something similar earlier which wasn't given as a foul which I sympathise with West Ham fans feeling it being a double standard. Personally I would argue both were fouls.
Keepers are in a unique position is cases like this because they lead with their hands which are obviously closer to the head. As such it is more common for outfield players to bail out of challenges like this on keepers to avoid injuries. Side note: This is partly what made the challenge from Stephen Hunt on Petr Cech all those years ago so horrible.
I am not saying these two incidents are the same and I do think Mendy overreacts which gets VAR to look at it. That behaviour is not unique to Mendy though. I remember Rudiger doing it against Son. And if Antonio had done it against James, James probably would have been sent off. That is where the game is unfortunately.
So those are my thoughts. Obviously the vast majority of people on this sub, or r/Chelsea, on MOTD and talksport to name just a few believe it to be a disgrace. So I am missing something. Please tell me where I thinking is flawed