It's a violation of Delta's rules but it is in no way fraud. There are things DL can do to you but having you arrested and thrown in jain is not one of them.

Also, mostly "back-to-back" doesn't involve throwing away a coupon but flying all flights but in such a way that it get's around having a particular night's stay (usually Saturday but doesn't have to be).

Some people seem to be worried -- having overlapping tickets itself is not a problem. It depends on the dates and fares purchased.