Knockoffs are not fakes so it is not against the law. Bag designs are the aesthetics, shape, form, materials and color combination. Over the years, most of these are not novel except might be some closure mechanism (for example, kiss locks in Marc Jacobs Stam bag was patented years ago).
Bag designs are always reissued or recycled; designers are inspired by each other to collectively create a trend for the moment. Of course, some lazy and uninspired manufacturing house will just copy the entire trendy design to the finest detail and ship it off to a developing country to produce it at low cost. This might be viewed as a rip off but it is perfectly legal.
I read about an interesting lawsuit filed by Hermès against Lederer which sold knockoffs of the Hermès bags in upscale New York Fifth avenue for years. The Lederer bags were exact replicas made in Italy with the same superb craftsmanship, quality and materials. Hermès lost the lawsuit as the logo was their only aspect of the design that can be protected by law.