Tridge's reverse engineering was why McVoy withdrew the license, but he'd started to restrict it well before that. Initial releases were source-available, and had a "fail-safe" that would turn the code fully open source if McVoy's company ceased to function. Source access and the failsafe were withdrawn over time, and various restrictive clauses were added. (Most notably, around 2002, McVoy added a "non-compete" clause which purposted to restrict any user for gratis BK from working on a competing SCM for a full year after they last touched BK. I'm not sure that was ever tested in court.) Here's a brief description of the history:

http://better-scm.shlomifish.org/comparison/bk-license.html