Assuming you have not purchased the plugin and taken steps in your build.settings to disable the splash screen:
Existing apps you have in the store will not show a splash screen unless you update them. After you update them, they will have the splash screen.
New apps you create will have the splash screen.
If you have apps in the store that do show the splash screen and you buy the splash screen control plugin, you would have to update your build.settings to disable the splash screen and submit an update to the stores at which point any one who updates to your latest version will have the splash screen removed.
This is a build-time decision as to if the splash screen is included or not.
Paid Enterprise subscribers and people who have purchased the Corona Professional ad plugin bundle get the splash screen control included.
Hope this helps.
Rob