Download the binary to a local store and have your script fetch it from there.
Why would the installer be an issue for the users? If this a script for your own deployment/installation what is suggested here is fine. But if you are recommending the use of Splunk to others, who will each be legally responsible for their installed instance then they should follow the defined procedure of logging into the Splunk site and downloading from the definitive source.
The installation process doesn't change. Users still have to accept the agreement the first time Splunk launches. All that changes is where the installer comes from.
Would this violate the license for other users when they don't have to acknowledge the agreement upon using the automated install?