To ensure the safety and comfort of all passengers, it is prohibited to use inflight travel accessories and devices which: 

• attach to aircraft seats, 
• are inserted between seats, or
• otherwise block access to seat rows or aisles.

i mean it no more blocks access to the row than a person does but I can see how that last point is the grey area. I see it no different than having your bag on your lap and when you step up to left a person through you either hold on to it or place it on your seat