Like Jon said the Forge will soon be more moderated. This means that when someone trys to take the premium content and re-upload it, it will not be allowed. They wont let you put it back up, and you most definitely will not be able to resell it.
Encryption of a file you are supposed to be able to use in your game and have complete control over is unnecessary and would be very annoying. Can you imaging trying to get a game working were you can't see the code in the behavior. You would have no idea what to do.
Or how about an un-editable tile set. what if you want to change a color in one of the tiles or really do anything. Locked actor, locked tile-set, locked behaviors, locked everything will do the developers no good. It would make the store much less attractive.
People who want to steal will steal but they will have to buy it legitimately first. A unique id attached to each store purchased item may be a solution but really there is little Stencyl can do about it with or without "mega ultra heavy duty encryption".
Other thread: There are restrictions on raising chickens in my town because they attract predators in to the city, creating even more dangerous situations (no chickens < child eaten by bear). Web sites that are trying to argue a point and are obviously biased will lie to you (or bend the truth as it is called). That is the point of their site, to sway you to their side by whatever means necessary.