Like what was said before the plastic plants Usually have sharp edges which can tear the bettas fins or scratch them or cause the betta to get wrapped into the plastic depending if they have long fins. And just in general They do not provide any beneficial bacteria to the tank.
It's better to be safe than sorry. If you don't want to get live plants, silk plants are the best way to go. Bettas like being able to swim in/among their plants so you want to be sure those plants are soft and won't hurt their fins.