Building on the Texas gulf coast and the builder used treated sills at exterior walls only --do the interior that come in contact with slab need to be treated as well

Yes. Concrete that comes in contact with the earth contains moisture. Wood that comes in contact with that concrete should be treated or have a moisture barrier installed.

Not Necessarily.

The idea is to prevent sill plates from rotting if exposed to moisture.

Preventing rot can be accomplished in several ways and still be code compliant.

One way to prevent the sill plate from rotting is to install a treated sill plate or use wood that is naturally rot resistant or a non-wood plate.

Another way is to install an untreated sill plate but separate the plate from the basement slab concrete with a water proof or vapor impermeable membrane.

Another way to prevent rot is to install a vapor impermeable membrane beneath the concrete before it is poured... and then no treated plates would be otherwise be required.