The question missed the version of SQL Server targetted, however, this behaviour is still confusing.

From books online:

However, the text will be available to privileged users that can either access system tables over the DAC port or directly access database files. Also, users that can attach a debugger to the server process can retrieve the decrypted procedure from memory at runtime. For more information about accessing system metadata, see Metadata Visibility Configuration.

(http://msdn.microsoft.com/en-us/library/ms187926.aspx)

Wouldn't we call viewing the execution plan a debugging exercise?