If I put out a buffet where some items are vegetarian and some are not, I don't advertise it as a vegetarian buffet with the excuse that the nonvegetarian foods are optional.
If you want to avoid Facebook with this library you can. That may not be the case with every library so folks who care about such things appreciate the callout. If you don’t care you don’t have to.
What’s the problem?
That’s just a buffet. When you use a qualifier like non/vegetarian, then the assumption is that the buffet would comprise of that.
As an apropos there’s a bomb vegan Korean buffet in Dallas.
I'm going to go out on a limb and guess you don't visit many restaurants that advertise vegetarian buffets