Why are drones thrown out of the hive in May?

It might be May for you, but the time at which the drones are evicted from the hive varies according to where you are. For me in the UK it is at the end of August.

Drones have only one purpose in life: to mate with a new queen, but this will only happen during the active (honey collecting) part of the season. When autumn/fall comes there will be no new queens until next spring. Honey bees don't hibernate, and the honey they made is their store of food for the winter. The drones would just be an extra drain on the winter food resources and their presence is of no advantage to the colony, so they are got rid of. More drones can be produced next spring.

Harsh, but that's nature.