This is not a bug. One of the requirements for an item to be a part of the semantic model is that is has one of the appropriate semantic tags. It would be nonsensical to have an item be a part of the semantic model while having no semantic information assigned to it.
The add point function is, by definition, creating an item inside a semantic group which means that item is going to be a part of the model which means that it must have a tag (in this case one of the semantic class tags instead of a location tag). The semantic property tag is option and can be set to none.
You can also check the “Show non-semantic” checkbox at the bottom of the model page. Then you will get an “Add Item” button below “Add Point” that will let you add a non-semantic item (including a group).
The checkbox is for temporary use only though, and won’t stay checked. There are also some bugs with the representation of non-semantic items.