Azure changes pricing tiers for databases automatically?

Question

I created a "Web App + DB" in Azure and deployed my ASP.NET API to it. For the SQL database I selected the "Free plan".

Now, after some days of running the API (only a dozen datasets were written and ~30 API calls were made) Azure/Microsoft changed the pricing tier from "Free" to "S0" and for another API from "Free" to "Basic". And now the database produces costs for every day.

Why is this happen? And how can I prevent Azure from doing this without asking me?

UPDATE: Some screenshots to illustrate the availability of free plans for sql databases in Azure.

Currently I've 3 SQL Databases with a free plan (how long? nobody knows) and 2 databases in Basic, that were former free plan databases:

Answer 1

Azure SQL Database allows you to create one free Azure SQL Server database per region, it reverts to Standard after 365 days. That is what I know ever since you use PowerShell to create it (I don't know about using other methods).

New-AzureRmSqlDatabase -DatabaseName $DatabaseName  -ServerName $sqlServer.ServerName 
-ResourceGroupName $resourceGroup.ResourceGroupName -Edition 'Free' 
-RequestedServiceObjectiveName 'Free'

This works only for the following regions, for now:

 Location           DisplayName
 --------           -------------------
 australiaeast      Australia East
 australiasoutheast Australia Southeast
 centralindia       Central India
 eastasia           East Asia
 francecentral      France Central
 japaneast          Japan East
 japanwest          Japan West
 koreacentral       Korea Central
 koreasouth         Korea South
 northcentralus     North Central US
 northeurope        North Europe
 southeastasia      Southeast Asia
 southindia         South India
 uksouth            UK South
 ukwest             UK West
 westcentralus      West Central US
 westeurope         West Europe
 westindia          West India
 westus2            West US 2