description says the fcg is full auto but does not mention "all nfa rules apply" like it says in the description for uzi parts kits.........can someone explain what and why they are selling these?

The only thing that makes it full auto is the safety selector goes to the FA position. Just about all of them do unless you buy the semi only safety.
The restricted item on the FAL is the FA ejector block.

Nearly all the parts kit have the FA lower, but as posted above, without the sear, they're useless.  

Quoted: The only thing that makes it full auto is the safety selector goes to the FA position. Just about all of them do unless you buy the semi only safety. The restricted item on the FAL is the FA ejector block.

Auto ejector blocks are not restricted. They can and are used in semi auto builds but are useless with out the cut to the upper receiver itself. Some builders and companies but a blob of weld in the safter sear area of the block as a CYA thing. Tapco was selling them and plenty of kits were supplied with them.
Generally most semi auto uppers come with a semi auto block installed but a few places were selling receivers without the block.

My Hess (yeah, I own one and admited it but bought it before I knew better) came with the auto block in it but there is no way to fit the safter sear in.

My FAL selector goes to FA too, but it'll never be able to fire FA (unforturnately)  It's all in the upper receiver (as already mentioned)