(a) it is trademark law<p>(b) trademarks are namespaced, both by business and geography. As a quick example, different Otto pizza chains ran peacefully in NYC and in Portland, ME. There only was an issue when they both tried to expand to Boston at the same time.<p>(c) missing vowels has nothing to do with collisions. I cannot create a company called flicker that hosts photos. I also cannot create a social network called facbook. Present or missing vowels do not make a difference. Missing vowels started with getting a web address, then became a fad.