Yes, unlimited bandwidth is indeed a marketing strategy. It is not realistic nor cost effective for any company. Bandwidth is an investment that every ISP or Web Host must make, and they are liable for the data transfer used on each server.
Especially on shared hosting environments, a host must allow for sufficient data transfer (as well as other resources, CPU, etc.) for all the customers on the server. Therefore, they usually insert rules into their terms/policies that enforce them to penalize a site (take it down for example) that is using excessive resources and tying up the server.
Much of the time, hosts that practice this strategy are usually gambling on the idea that most sites will not actually use much data transfer for their account at all, meaning that they never actually take advantage of the unlimited offer - if even a few sites on one server greatly increase their usage - the server is more likely to crash altogether.
If you are adament about choosing a host with unlimited data transfer, I suggest that you carefully read any SLA or use policies they have.