Personally I find it puzzling that Omni Hobbies, also owned by Tower, ships free for orders over $100 (as do others) without having to join/pay anything.
I think the Tower Hobbies owners like to experiment with different business models. Omni Hobbies doesn't charge for shipping on large orders, but, on the other hand, they don't send out piles and piles of advertisements every month, including many $5 and $10 off coupons. They also don't have Tower Hobby "house brands". As a result, it costs almost exactly the same to shop at either, which I, for one, find extremely annoying.