I actually think I can see where Neonivek is comming from. When you get a gift, it does require you to respond and act socially in a particular way. It also comes with the expectation that you purchase presents back at some point (otherwise your greedy, or on general people look down at you). Some people just don't want to be part of that, and I imagine for these people it could be frustrating explaining that they don't want to receive gifts and have people assume the opposite.
It is more of the "act socially in a particular" way that is the issue. The whole "giving a gift back" aspect isn't an obligation nor is it really a big deal. At most it is a pain rather then a "horrible thing to do to someone".
The reason it is selfish to give someone a gift they specifically asked you not to give them. Is because giving them a gift FORCES them to ingratiate themselves and to accept the gift.
You are using gift giving to lock them into a situation where they have to "Like you for it". You are doing it because of your selfish desire to be a "good gift giver" person for lack of a better word and for the words of thanks and praise you will receive for doing so. It cannot be because you genuinely believe they want the gift because they explicitly asked you not to.
The problem to me isn't the yelling, it is that even without the yelling no one would take her side. This board would still be thinking "That bitch" if she said "I don't want them" or "Take them back" or "I cannot accept this gift" or if she throws it out because when she receives a gift she is OBLIGATED to like it... Ohh and if she refuses to use it and she even touches those flowers it makes the guy "Right" for buying them against her wishes. He is basically buying her with that gift.
Literally the only thing she could do in this situation that would be socially acceptable is to graciously accept the gift and "enjoy it"... and that is why it is so terrible to give someone a gift that they explicitly ask not to receive. That is it... Nothing else... and why I don't consider her the one with sole "in the wrong" for being so emotionally soured by it that she resorted to yelling. He is in the wrong too, in fact more so. It is outright manipulation using social convention.