Why is determining existence (or not) of rows matching certain criteria using a 'LIMIT 1' query on indexed columns examining more than 1 row?
LIMIT provides a window into the result set.  LIMIT 1 is equivalent to LIMIT 0, 1; return 1 row starting at offset 0.  Explain plan reports the estimated number of rows the LIMIT is running against.  However, only one match should be examined. 

Testing on your SQL Fiddle shows the query runs against the INDEX (KEY) which should be the fastest access method.  I have found queries of these form are sometimes faster than queries returning a constant.

SELECT owner_id
FROM someTable s 
WHERE reviewed = 0
AND owner_id = 0
LIMIT 1;


I would check the I/O on the device(s) storing your data.  I/O contention can cause issues on larger databases. 

I would also consider a query in the form:

SELECT MIN(event_id)
FROM someTable s 
WHERE reviewed = 0
AND owner_id = 0; 


With an (owner_id, review, event_id) indexed.  This should run against the index and examine only one row of the index.