It is common among unscrupulous companies to bring someone in for free advice/work under the disguise of an interview. The technical screen for which they hauled you in was a trick to get you to do free work for them. You gave them software solutions that worked and worked well. They would have had to pay hundreds, if not thousands of dollars to a consultant to do just that for them. I hope you learned your lesson. Sorry that it had to be the hard way.