There is zero evidence that Erdnase's book was seen as "obscene" or illegal by anyone when it was published. Quite the contrary - it appears to have been completely ignored! Only a few magicians seem to have paid any attention to it although it continued to sell well via gambling supply stores for 60+ years after it was released.
The idea that he might have had problems publishing the book due to the "Comstock laws" that were on the books is a theory put forth by Hurt McDermott. It's an interesting idea, but supported by no hard evidence. To the contrary - there were numerous other books on the exact same subject published for decades before and after The Expert at the Card Table. None of them seemed to have any problems with the authorities. Why would Erdnase's submission to the genre be any different?
There is also zero evidence that cheaters or magicians disliked the book (or author). Again, the book was essentially ignored when it was first released. It was primarily (although not only) due to the championing of the book by Vernon, Downs, and Miller that it gained any traction at all in the magic community in the early 1920s. As for cheaters, most of the hard-core hustlers of the past 50 years or so will tell you they never heard of the book! That may not have been true in the first half of the 20th century - after all, someone was buying it quite regularly from the gambling supply houses (as I stated earlier), but it doesn't appear to have had much of an impact on cheating one way or another. Cheaters still operate virtually identically to the way they did before Erdnase was published - collusion, paper (marked cards), holding out, cold decking, false dealing and stacking. All of that was known and practiced loooong before The Expert at the Card Table was published.
To cheating and cheaters the book was little more than a blip on the radar. To magicians it's primarily remembered as the book Dai Vernon told us all to read. It's value is considerable to us from a historical standpoint (for a variety of reasons), but it didn't affect the gambling world much at all.
Jason