The problem with what you said, is that in the game, Suzy and Tammy are not in a relationship when you start the game, in fact, up until almost the choice of throuple or nothing, there isn't really much that points that their relationship was something serious. Also Suzy declares her love for the MC before that even happens.
In fact you have said yourself that Suzy's story is about her being in love with two people at the same time, so it's no wonder that people that play the route of Suzy and Tammy the first time, would be shocked to see her changing from I love the MC to I love Tammy more (because she will always chose her no matter if the MC is included or not).
So saying the story of the characters is about them and that you can't change who they like, is a bit weird when both characters wants the MC and Suzy expecially is in love with the MC and later Tammy, said even by yourself. So if it was IRL there would be chance of one of them, choosing you over the other, instead of we decided we love each other now join us or be gone, it's not realistic, based on the fact that Suzy is said to be in love with the MC. But since you forced that, I don't know why you are surprised by people who play the game and encounters that throuple choice after playing during a long time and got surprised with no choice to choose only one of the girls, the throuple or no one, instead of throuple or alone.
I mean you have to play their routes during a long time to suddently have the throuple choice coming out of nowhere in the game, I mean there were not many things pointing to the fact that they love each other way more than the MC, also there was nothing in the game that developed their relationship more than they like to have fun with each other, no deeper feelings where expresed anywhere in the game between Tammy and Suzy until almost before the choice, at least there was nothing the last time I played, so it kinda comes out of nowhere if you download the game without reading anything about it.
Just to nitpick, I can like a character and said character be my favourite without liking everything about it, you can like the character itself but not the choices the author forces him/her to do or even to not like a part of its personality, so to say that if you don't like everything about a character it can't be your favourite was rather dumb IMHO.
