IMO, the workplace is not a place to display, discuss, promulgate, etc., one's political, religious, or social views. Certainly if one is the OWNER of the business, one may do this, but at one's own risk (of losing business).
Now since United's employees own 55% of the common stock, they may feel entitled to do this type of stuff. However, since no ONE employee owns a majority of the shares, I believe such a feeling would be misguided at best.
Having said that, this particular display would not bother me, personally. Nor would it bother me if an employee wore a yarmulke or crucifix, etc., on the job. If they were to start TALKING about their beliefs, then that would bother me. If a particular airline started putting religious symbols on the uniform, that would bother me, largely because it might have the effect of forcing employees who believe something else to comply to save their jobs. I don't fly Alaska Airlines and was unaware of the proselytizing napkins, but that would bother me.
Dunno if that's rational or consistent, but that's my $0.02.