Maybe, if you could kindly explain what you mean by that, because I don't get it.
the bigger problem with discriminating based on employer is that employers are very large entities. We cannot expect everyone to know everything their employer is doing, let alone be responsible or actively contributing to it
There are over a hundred hours of Adolf Eichmann trials on Youtube. Watch any one of them, provided it contains him defending himself. So if the bigger problem is a complete non-issue, what does that say about the smaller ones?
Also, as dalke points out, there is legal precedent precident that discrimination based on employers is discrimination (specifically the ACLU kicked out a non-uniformed unannounced police officer). Granted this was only a violation of (California) state law, but the law had a specific list of protected classes, and employer was not on it.
What is legal or not might be an issue for the organizers, granted, but personally I care more about what is right and what isn't. So if they break the laws for this, more power to them; if they can find a loophole, also fine. Private clubs can invite whoever they fuck they want, for example; this wouldn't be very practical, but there's nearly always a way.