I wondered that myself some years ago about that very tree.
In all my Gleditsia triacanthos 'inermis' seedlings they never got a thorn as 3 - 4 year seedlings anyway. I never found a thorn on them. I'm thinking "wow, what an economical way to grow this cultivar". And it was one of the easiest to get to sprout and grow.
I had always heard the same thing, and its probably true in most instances that cultivars don't come true from seed.
That must apply to some cultivars of plants more than other cultivars of plants. My guess is 90% of cultivars will look not much like the real cultivar.
Take the Japanese Maple 'Bloodgood' for example, I learned some nurseries grow it from seed and I believe its purple sometimes but green the rest of the time, ex. when its hot in the Summer its green. When I learned that I thought, "what a ripoff". There people are spending a ton of money for what they think is real 'Bloodgood', and its a fake.
Really, all you usually have to do is ask the nursery boys or girls if they're grown from seed or grown by cutting.

Hmmm. Have heard the same thing. A few observations:
--- Your seedlings may not not have been old enough to display thorns. Think of how they are on older trees...
--- If the original tree is truely Gleditsia triacanthos variety inermis, and it produced seed from self pollination or pollination by the same variety, then by the botanical definition of "variety," seedlings should be thornless too. However, if it is Gleditsia triacanthos 'Inermis', as I was originally taught a loooong time ago, then (it being a cultivar) seedlings will most likely be thorned.
--- From a quick google search, it seems to be the general consensus now, that, inermis is a variety, and not a cultivar, as I was originally taught back in the '80s. So what we've heard about seedlings being thorned is probably just a continuing myth from back then, as varieties breed true for the differentiating characteristic of being thornless. Clear as mud?

Honey locust seedlings will get thorns the first year. Probably what will determine thorns or no thorns is how the genes shake out and odds would change with cross pollination or self pollination. I have a thornless one I left in a fence row but it produced no seeds this year.

While I realize this is now an old thread... I had read I_james (mo5)'s comment last year and was disappointed that most of the thorny honey locust seedlings I am growing were still thornless after a year of growth. It's now the beginning of year 2. With the new growth many are getting thorns! So, it might be a little more than a calendar year before they start getting thorns.

I'm wondering if some thorns on the branches are going to be that big of a deal? Many ornamental plants have thorns and isn't it the clusters of large thorns along the trunk and larger branches that are the bane of Locust trees?

I have some sprouted from seed last year but I figure it will take several years before I'll know if it's going to grow like a species tree.

Funny, where I got my seed from (N. Illinois) doesn't seem to have a problem with thorny locust growing wild, even though there're large thorn-less varieties growing all over the place.