I tested it with following steps and see that the Read-Only functions as expected. 

1.  Created a New Custom Attribute of type Integer in Task Work Item from Scrum PA
2.  Added a presentation for it with
-  Kind : Integer
-  ReadOnly : true (modified it from the default "Inherit from Attribute")
3.  Created a work item in Eclipse Client and I cannot add any value to the field added in step 2
4.  Opened the same work item in Web Client and see the same behavior - I cannot edit it.

If its the same steps you follow and still it fails to maintain read-only, I would suggest raising a ticket with Support with your observations on any PA created using OOTB process template.

Further, I reverted the attribute presentation to read write by modifying ReadOnly to false and I was able to successfully edit the value.

Later, I modified attribute to ReadOnly and kept the presentation to Inherit from attribute.  I wasn't able to edit.
with attribute ReadOnly, I modified the presentation with ReadOnly:false, the presentation for the field shows up greyed out and again I was not able to edit.

So, since my observations show expected behavior, what we are seeing is more possibly a configuration issue rather than a defect.  Please review your configuration again.