Unions are member-supported organizations with finite resources. (Really, national and international unions have basically infinite resources, but the business owners have finite resources to allocate in order to deal with them.)
The right to an attorney are with respect to criminal proceedings. A defendant does not have a Sixth Amendment right to counsel in any civil proceeding, including a deportation hearing. A fair trial is something that should be available to everyone, but if the lawyers are bogged down defending cases that their own members don't support, then either those unions or those business owners are doomed. I understand this to be a deliberate consequence of union formation, the threat of force is truly a compelling bargaining chip.
Many employment agreements are "at-will" and employers can terminate their employees without cause. In the case where such a firing is really "without cause," I agree that a union should support their member by representing the aggrieved party and making sure they get their unemployment benefits, but if the law says the employer has the right to terminate the employee for any reason or no reason, then this policeman can justly be fired with a laundry list of complaints -- that dwarfs the complaints for every other officer in the precinct -- and a last straw, is this constitutional violation, bullying a person off of public property for taking some pictures of public officials "doing their job."
If I were the union steward, I would meet with the officer well in advance of the hearing, before it had even been scheduled. I would have to open with "OK, convince me."