Hi there,

The cabinets created with the Cabinet Tool (Floor tab) are components, therefore as most components (such as doors, windows, etc) they can be added only by attaching them to the walls.

Alternatively, if you are interested, I created several collections of cabinets as 3D objects so you can add them without walls.

Check these collections
Cabinets Collection
Kitchen Templates Collection
Create your Own Cabinets (this collection was released yesterday).

Hope this helps.