The mechanism of selenium framework:
Here selenium make request to the its server and fetch first subelement with tagname input
WebElement e = e.findElement(By.tagName("input"));
Than you try to receive attribute on that element
object attribute = e.getAttribute("checked")
So either use
findElement(By.attribute("checked")
or use
findElement(By.xpath("\\input[@checked='true']")
P.S. I'm not familiar with java's equivalent of selenium api so some method may be named slightly different.