It's not "spare" housing. People who buy apartments for use with AirBnB are reducing the housing stock available to people who want long term rents or to buy to live.
Bad analogy. If the grocery store runs out of celery it doesn't have anywhere near the impact on your life as not being able to find a home reasonably close to other important things.
Perfectly good analogy. Not living in the exact place you want does not cause unreasonable suffering ("nearest starbucks is 2 miles away, oh horrors" ?), so it should be subject to market forces.
