Full perm, short for full-permissions-which-includes-copy-modify-transfer-ability applies to an item that is sold with these permissions. If it happens to be 'free', meaning no cost to buy, it means that you received for no payment an object that you can take apart, modify, copy and also pass on to someone else, either in its newly modified state or not. The fact that it was free and full perm does not mean that other objects that are 'sold' for no charge (free) will also be full perm.
Most builders who wish to sell (for free or otherwise) also want to preserve the integrity of their creations. They may choose to remove the permission to modify and leave copy and transfer, meaning that the state of the object cannot be changed by the buyer, but that it can be copied in its original state and also transferred to another user at no cost. This is the most usual state of permissions on objects put up for sale and for good reason. It allows everyone to enjoy the commodity while preserving the integrity of the builder. The builder should be the only user who can 'sell' an object although rules for this may vary across grids.
The reason for object permissions is often either not understood or misunderstood. It takes a bit for a noob to get her or his head round what the 'perms' actually mean. It gets even more complicated to understand when objects have contents because the contents of an object can also have permissions and they may not match the permissions of the object. This is most often an issue when an object contains a script with a different set of permissions. An object may have full perms but be still restricted in what the owner can do with it if it contains a script that's not full perm. No Copy is usually the permission that causes the problem and confusion as to why an object with full permissions cannot be copied when a script it contains is set to No Copy.
So, to end this long-winded soliloquy (which I apologise for) you are not stupid if you expected a 'free' object to be one that you can copy, modify or transfer. You just need to remember about permissions. If your 'free' object is full perm - no problemo.