The harm side comes in multiple forms:

Harm to the animals; by removing their nutrient dense food source, and feeding them sugar water in its place, impacting colony health

Harm to the ecosystem; by mass producing honey bees we are choking out other pollinators, and the selective breeding for honey bees prioritizes output and makes colonies more susceptible to disease and collapse.

Even if you feel like the bees we're farming lead a good life, that life comes at a cost of other species - we are choosing a winner in the food web in a way that could be done less harmful for similar end result (i.e., plant sugars / syrups). Much of veganism is about harm reduction.

Knowing the importance of pollinators to our food supply, as a vegan I would probably not have much of an issue with pollinator farming if there goal was maintaining biodiversity, instead of min-maxing profit.