I think you are confusing the term veganism with WFPB.
The definition of veganism by The Vegan Society, who originally came up with the term.
"Veganism is a philosophy and way of living which seeks to exclude—as far as is possible and practicable—all forms of exploitation of, and cruelty to, animals for food, clothing or any other purpose; and by extension, promotes the development and use of animal-free alternatives for the benefit of animals, humans and the environment. In dietary terms it denotes the practice of dispensing with all products derived wholly or partly from animals."
What is a vegan and vegan food? What does veganism mean?
www.vegansociety.com
It is up to each individual to decide if they want to eat junk food, or not, and regardless of which one is eaten, the focus of being vegan and not harming animals, will still be accomplished. Vegans, by their very nature research things, and if they want to eat some junk food, it's more than likely they know what is in it and if it's unhealthy or not and are making a decision based on what they feel is right for themselves.
Figuratively slapping junk food out of someone's hands is not helpful, and is what gives us the "ridiculous, crazy vegans" title.