You're right. Several of the tweets included that tag. However, it doesn't seem to me that those tweets were from people who were thinking these shirts are targeting women. If you take a look at the hashtag on Twitter[0], many of them have nothing to do with boycotting an item. Is it safe to assume a person would see that hashtag and use it thinking it isn't specific to sexism? I think so.
I should also clarify that I am in no way arguing that rape is equally a male and female problem. Women deal with rape culture much more than men. My argument is that I don't believe these twitter users were upset because it was targeting women. I think they were upset because rape is a terrible thing to happen to anyone.
[0]: https://twitter.com/search/realtime?q=%23NotBuyingIt