When looking at the Salesforce edition comparison chart, it states that Integration via Web services API is only available if you pay for it.
Indeed, when I try to load data into my org using Apex Data Loader, it does not work, since I have no API access. However, Dataloader.io and Jitterbit Data Loader do work with my professional edition, which is not what I would expect.
I see no technical reason why these tools work? Is there maybe some commercial agreement between them and Salesforce to skip this API limitation when the request comes from their application?