This has been discussed quite often on these forums. The basic short answer is no.
However to put it in context , anyone can easily extract the assets from all games out there whatever format they are in. Some game packages may take a few steps more than others but ultimately nothing is sacred.
Edit (To Add)
Ive noticed some of the Steam nwjs packages are just downloaded directly to your PC so you can do whatever you like with them.
But other nwjs packages are not obviously accessible or visible and need to run through the Steam Client.
I've not released a game on steam so dont know how this is determined but obviously the latter would be preferable if you are looking for extra deterrent. (Though im sure the package must be buried in the Steam inner file system somewhere if you really wanted, never bothered to look though)
Maybe someone who has released a game on Steam game could enlighten us?