Maples do produce pollen and nectar. If you’ve been around maples it’s likely you’ve noticed the “flowers†from the maples that have those “wings†on each side lying on the ground. In spring the willows also bloom and bees will work those for pollen, too. As has been pointed out maple sap has a small amount of sugar in it, but it’s pretty diluted and I’m inclined to think bees would be more apt to go for actual nectar over diluted sap.
With that said, back on the farm in upstate New York where I grew up, we had a few bee hives and produced a small amount of maple syrup this time of year. On nice days we’d see a few, although not a large number, of bees landing on our sap filters, etc. When we were evaporating, some bees would hover around the steam that came up from the evaporator. At the hives, many bees could be seen going back and forth. More than we’d see at our maple operation. So, what were they gathering? I’d imagine most bees are gathering actual nectar from certain species of trees, including maple.
(Edit: Maple syrup is produced simply by boiling off excess liquid. When the water is being evaporated the syrup is not actually "carmelized" as some people maintain. Finished maple syrup contains a fair amount of minerals including potassium that probably would not be good for bees.)
