Actually it is Title 2, discrimination in a public accomodation.
There are a "few" states, under thier constitution's free speech clauses that permit, not demonstrations per se, but the distribution of literature, etc., (specifics here) on private property, such as a Mall, regardless of whether the Mall desires it or not.
Compare this to the other thread on this forum wherein a person tried to leave the store with a cartful of merchandise directly to the door from some location other than the checkout line, and "couldn't find" their receipt. Looking from the store's perspective it's easy to see that a "no thanks" and "I can't find it" are just excuses for someone who never had a receipt in the first place. One was shoplifting, the other wasn't? How does the store tell which is which?
If the OP feels there is a case to answer then they should pursue that case.
In your particular case, declining to show a receipt does not give the retailer "reasonable cause" to detain and treat you like you have written. I suggest you sit down to talk with an attorney in this matter who is well versed with: 1) retail security policies, practices and procedures, and 2) illegal detention.
If you take the time to think about the two cases, it should be obvious to you that in the former case the person was stopped on suspicion of shoplifting, and in this case the person was stopped for having the audacity to try to leave the store after making a purchase.
If you just want to bicker, take it over to a social forum.
A problem in terms of interesting a lawyer in litigating is that the damages don't appear to be substantial. (That may be something the store took into consideration when it set its policies - "we can do this because people won't sue.") It's also a bit esoteric - in my experience cases involving mere detention by store security, as opposed to physical injuries, aren't often litigated.
The Arizona state bar offers a lawyer directory, but they do not appear to offer a referral service. You could also try the AAJ directory.