1. Sprit Airlines charges even for carry-ons and they made a new rule completely separate from other airlines in order to confuse and prey on the hapless customers. Their current free allowance is for a "personal item" of size 16"x14"x12" which most suitcases in the market will not fit. Their check-in baggage weight is 10 lbs less (40lbs) while other airlines are 50lbs. I think this is intentional fraud to confuse and prey.
2. This rule is also not enforced all the time and is left to the discretion of the employees at the gate and the counter. As one can understand they heavily discriminate on their decision. You can run a hidden camera operation to prove this.
3. Our luggage fit their measurement cage and they allowed us to travel one way (DTW to NYC) but charged us on our return journey. I have attached photographs of our luggage in their measurement cage. It is above the cage a bit due to the foot at the bottom of luggage.
4. Ticket counter agent at Boston threatened me with only two words which she kept on repeating "It is $40/$45 or no boarding". We also spoke to her supervisor (who wouldn't give his name) to no avail ("I don't want to loose my job for 1"). I told her that I will take my chances at the gate agent to which she replied "I will be at the gate"-which turned out to be a lie. At the same time we saw many many people using their regular sized carry-ons (22"x18"x10") and not paying anything. Almost no one stowed their "personal tem" under their feet.
Review about: Spirit Airlines Baggage Policy.