Not able to "Clear" notifications from Astrid Tasks (an app)? I am not able to clear notifications in the status bar for an app named Astrid Tasks.

Here is the screenshot: 


Why is it? I am sure that the app developer must have done this on purpose.

What should I do to get it back? 

Can someone give me a link where I could read about these practices?
Astrid Tasks uses notifications that cannot be cleared by clicking on the "clear" button. Why they did this is a question only the Astrid developers can answer, but my guess is the people from Astrid want to remind you of a task until you actually finish it and/or change its status in Astrid. 

By default notifications on Android can be cleared, but an app developer can choose to make a notification not-clearable (if you want technical details on how to do this, go here). If there are any notifications that can be cleared, the clear button is enabled/visible but will not clear any non-clearable notifications when clicked.