Observations
I am not sure what "trespassed out of" means but you had already left the store, nobody asked you to leave nor were you charged with trespassing (which you could have been - be observant).
Even if a sign does not meet the statutory requirement, if you know that firearms are not permitted (e.g. by recognizing the intent of the sticker or from prior experience), you still may be tagged with a trespass charge. I know that you did not see the sticker in this case but don't listen to those you say you can ignore a 4.99 x 6.99 inch sign with impunity.
Comparisons to discrimination on racial (or religious or gender or other protected class) grounds make you look silly. There are specific statutes that make such behavior unlawful. It doesn't win the pro-carry movement any points to say that being barred from a retail outlet because of your pistol makes you a lunch counter sit-in equivalent. Focus on the "loyal customer" relationship instead. It doesn't make the merchant feel put upon and highlights his self-interest.