Perhaps because they have been or already are developers. Any good developer is already a tester, engaged in Test Driven Development, using xUnit family of tools, running their tests continuously with CI.
If he's on the ball he's using Behaviour Driven Development to script complex scenarios that function as both his development and test harness.
He's testing his Web Services integration with tools like SOAPUI + Groovy from Smartbear from the article and using it to do dynamic mocking for both the client and server.
He's targeting the end system, using tools like WebDriver to execute his test scripts using all the major Browers, IE, Firefox, Chrome, Opera and Safari.
If he's doing all that and running his tests from the cloud to load and performance test the SUT; he might just have the technical skills to match a typical automated test engineer.
Anybody that thinks testers today are just using the Application is frankly cross ignorant of what is happening to automate the test specialism.
Test automation is the undiscovered country for developers that have done it all before a dozen times.