Sometimes straight cisgender guys have TG partners or TG children and want to enter a workplace that will offer desirable health benefits for the entire family and want to be able to bring such family members to any work-family functions. I can't think of a better event to attend to find such places of employment.
Of course, in the case of this individual, it may be that he merely seeks a tolerant place of employment (and wants to make sure management doesn't permit anti-GLBT bigotry), or just that he wants a job at all. However, even in a worst case scenario, it shows that he is acknowledging TGs. To be welcome at the event, he at least has to be civil, and it is a basic exercise that helps integrate TGs and non-TGs together. Susan's Place has a substantial share of non-TGs as well, and the site takes pride in not excluding them on the basis of their non-TG-ness, so it's not an precedented idea.
In some ways, being TG isn't particularly clearcut or obvious either. Let me borrow from our Wiki. One of the three conditions that can qualify a person as TG is being "...a person whose identity does not conform unambiguously to conventional notions of male or female gender roles, but combines or moves between these." This condition is broad, perhaps intentionally so as to avoid being exclusionary, but it doesn't take much to "not conform unambiguously to conventional notions of... gender roles." A second way of TG not being so clearcut is with post-op Transsexuals. Even though our Wiki definition includes post-ops under the TG umbrella, the main idea of transitioning is to be that target gender, not be a TS of that gender. Post-ops definitely need to have TG-friendly employers, since background checks can turn up their prior legal sex, but it can be uncomfortable for some of them to claim being TG when they've worked hard to move past being TS. I remember a member here having a signature to the effect of "I am a woman. I am not a trans anything."
Given all these points, there is merit to broadening the scope of a "TG job fair" to a "TG-friendly job fair." Given the event's roots and mission, TG support information needs to remain an important component of the job fair, but there is a lot to be gained by making the event open. And really, the job fair probably is this way already. Besides, as other people pointed out earlier, there are plenty of TG people who can pass flawlessly as a non-TG person, so it's not really feasible to turn away people for not being/looking/acting TG.
Those are my early thoughts on the matter.