Microsoft has heard the call of 6870 votes and 300 comments in 9 days, putting this request second on the Office 365 Admin Uservoice list.

An update was done to the FAQ yesterday :

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UPDATE as of October 31, 2019: Over the past week, we’ve been listening to customer feedback regarding the rollout of our self-service purchase capabilities for Power Platform products. Based on the feedback, we’re making the following changes to our plan:

• On November 19th, we will provide IT admins a way to turn off self-service purchasing on a per product basis via PowerShell. More details will be forthcoming.
• To provide more time to prepare for this change, we are updating the launch for self-service purchase capabilities for Power Platform products to start with Power BI on January 14th for all commercial cloud customers.

Beginning January 14, 2020, self-service purchase, subscription, and license management capabilities for Power Platform products (Power BI, PowerApps, and Flow) will be available for commercial cloud customers in the United States. Self-service purchase gives users a chance to try out new technologies and enables them to develop solutions that will ultimately benefit their larger organizations. This capability will not be available to tenants in the US that are government, nonprofit, or education, at this time. Central procurement and IT teams will have visibility to all users buying and deploying self-service purchase solutions through the Microsoft 365 admin center and will be able to turn off self-service purchasing on a per product basis via PowerShell.

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Now while this is a start, it’s not ideal. We believe this should be off by default with an opt-in radio button option on tenant level to make things easier for companies. Well done Office 365 / SharePoint community for speaking up, we have made an impact. Don’t let up now, add comments to make this easier for companies to manage instead of having to always resort to PowerShell.