It's legal if the fare rules says it's legal. Of course, NW might check it out anyway as one of those 'suspicious' itineraries.

BTW, most lower price excursion class Y class tix are not circle trip material. What I mean is just because a fare LAX-LGA permits connections in MSP/MEM/DTW/IAH doesn't mean that if you manage to go through all of them that it's automatically a circle trip, but rather a simple return ticket with allowed transit points. A circle trip is defined as a trip with more than one destination, such as LAX-MEM 01Dec, MEM-MSP 04Dec, MSP-LGA 07Dec and LGA-LAX 10Dec. This is why the four hour rule sometimes comes into play on MR as the airline considers anything longer to be an intentional break of the journey and so recalculates the fare as a result.

Of course as we all know with airlines, there are no hard & fast, black & white rules that apply in all situations. You may find seat sale K class fares that can be combined and will create a circle trip itinerary for a reasonable cost. More often than not circle trips get bumped up to more flexible, generally higher price fare basis.

Hope that explanation helps.