If you add the code I noted it should stop the script and print the query. that is what exit does. Everything I tell you I tried on my site before I posted I verified what is ran which is why I linked my site which is a clean install. Like I said if you hit all review the what variable is comments which means the only table searched is reviews. The products table is not searched which is why I asked about mods since I know you modify things normally. If I search on my site and hit all reviews and leave keywords blank it returns 21 results which is the number of reviews so again it is working correctly. Like I said hitting all reviews only means the reviews table is searched.
Here is what I get searching your site