Free = OOB connections Standard adds Custom API Connectors Enterprise adds On-Prem + Management
Enterprise subscription is required to deploy CustomAPIs inside of PowerApps AppService Environment. That means Standard offering limits subscribers to only those connectors provided by MSFT Out of the box. This is doesn't provide enough value for a paid offering, IMHO. The OOB connectors do not meet many needs and discourages partners from expanding the available APIs.
The potential for PowerApps reaches way beyond employee facing apps inside large enterprise orgs. Independent developers are publishing APIApps to Azure and these need to be able to be easily discovered and deployed into any orgs PowerApps tenancy.