55 gallon walmart tank, filtration question

I bought the Wal-Mart kit and apart from the filter I thought it was a good deal. That junk Aquatech filter is still on the tank now that I've converted it to sw, but just as a water pump (the cartrage is quite simply the worst design EVER). What I don't understand is why Wal-Mart didn't put the Tetra brand filter in the 55 gallon kit, since it's cartrages are easy to rinse out and are therefore reusable, and have more bio surface area anyways...
 
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