I would fire them too, with these reasons:

1. The interns saw a dress code violation and escalated immediately, without informing the out-of-line employee.

From the original blog post [0]:

"I felt the dress code was overly strict but I wasn’t going to say anything, until I noticed one of the workers always wore flat shoes that were made from a fabric other than leather, or running shoes, even though both of these things were contrary to the dress code.

I spoke with my manager about being allowed some leeway under the dress code and was told this was not possible, despite the other person being allowed to do it."

I'm certain the interns did not ask this coworker why they came in wearing non-standard shoes, because management had to tell them about the disability after the fact. Going directly to management to censure a coworker without discussing the issue with the coworker is sometimes justified. Minor dress code violations are not an instance where that behavior is justified. If this isn't apparent to the interns, that raises questions about their judgment.

2. The interns persisted in making it an issue after the first rejection.

Again, there are some workplace situations where it is appropriate to keep raising an issue after your first proposal is rejected. Minor dress code violations are not one of those situations. Again, from the blog post:

"I spoke with my manager about being allowed some leeway under the dress code and was told this was not possible, despite the other person being allowed to do it. I soon found out that many of the other interns felt the same way, and the ones who asked their managers about it were told the same thing as me. We decided to write a proposal stating why we should be allowed someone leeway under the dress code."

The proposal which led to their termination was not the first discussion of the issue. They asked, management considered their request and came to a decision. They then took more time (presumably out of the workday, though this is not clear) to write a petition restating the request. Continuing to focus on a very minor issue at the expense of more important things is another strike against their judgment.

3. The interns asked for much more leeway than the original infraction.

The coworker who inspired their petition wore non-standard shoes, hardly a large violation of the dress code. In response, "[The interns] requested that we also be allowed to wear running shoes and non leather flats, as well as sandals (not flip-flops though) and other non-dress shoes that would fit under a more business casual dress code. It was mostly about the footwear, but we also incorporated a request that we not have to wear suits and/or blazers in favor of a more casual, but still professional dress code."

Seeing one employee wearing non-standard shoes is not an endorsement of a less-stringent dress code by management. It is odd that they take the dress code so seriously, yet take the example of one employee not wearing leather shoes as a sign that the dress code can be relaxed to exclude suits. That hypocrisy on the part of the interns is at least as bad as any perceived hypocrisy on the part of management.

All of these lapses in judgment are bad enough to warrant censure. If an employee did this, counseling would be in order. But interns are explicitly not full-time, salaried employees - it is a trial run. (I don't take Alison Green's view that they are guests, but they aren't full employees either.) If the company's goal in having internships is to evaluate potential employees, then a lapse in judgment like this might mean that the company's interest is to let them go before the official end date.

[0] http://www.askamanager.org/2016/06/i-was-fired-from-my-inter...