The people setting the wedding dates are the fiances, who are the ones organizing it and are also the main recipients. In that case they are both the vendor and customer, it is pretty much like a DIY situation, even if there is some outsourcing involved.

Now I won't say deadlines are bad, but they shouldn't be set and promised without first getting a careful estimation, which is what the parent comment is criticizing.

- deadline set randomly or by gut feeling of a manager = bad (more often than not)

- deadline set after careful estimation of resources involved, their own estimations and possibly the number of side projects that might have to be rescoped/paused for that purpose = good