The Splashtop Streamer application will only be installed if Splashtop is enabled in at least one policy the device is in (and all parent policies if the device is in a child policy). This behavior is the same for devices in Flex policies. If a Parent Flex policy has Splashtop enabled it will be deployed to the device. If the Flex policy is a child policy all of the parent policies above need to have Splashtop enabled for Splashtop to be deployed.
Disabling the integration in a Child policy will override the state of the Parent policy. Thus, enabling the integration at the Parent level does not automatically enable it in Child policies.
Child policies have Splashtop disabled by default - it must be enabled for each individual Child policy.