Why can a non-clustered index not be dropped using online=on option

When attempting to drop a non-clustered index using the online option such as DROP INDEX [IX_MYINDEX] ON [dbo].[myTable] WITH ( ONLINE = ON ), I receive the following error message.

Msg 3745, Level 16, State 1, Line 16 
Only a clustered index can be dropped online.

The SQL Server documentation clearly states:

The ONLINE option can only be specified when you drop clustered indexes.

http://ift.tt/1GJ8eob

But can someone please explain to me why this is the case? In my experience, you would be much more likely to drop a non-clustered index than a clustered index, since the clustered index in most cases is also your primary key.