Emperor were a distinctive outfit with a sound which no one has been able to successfully imitate. They're "controversial" because they were an authentic black metal band who popularized black metal. Their influence, unlike, say, Mayhem, was primarily musical and so it was easier to point to Emperor as causing an influx of posers. Immortal's influence was felt here also but Emperor's was greater. Remember that the popular face of 'extreme' metal had used to be swarms of interchangeable Suffocation clones and in the mid 90s, after the Norwegian explosion, it was a horde of third rate "symphonic black metal" with the signature Emperor synthpad+shriek aesthetic.
Postscript: Most 'underground' 'extreme' metal fans ultimately buy into the genre because of the promise of individualism. In other words, metal gives them a guarantee that they stand out from the crowd without them having to do something inspiring and thus risking failure. It fulfills a need for a "unique" identity among low self-esteem people by providing them with a massproduced, prepackaged one marketed as "esoteric" and "rebellious". As suggested, it also guarantees them membership into a community of similarly insecure social misfits. This is honestly the main reason why metalheads instantly look upon more popular acts, or bands that grow to popularity, with suspicion and accompanied hostility: because it abstractly threatens to strip them of their inflated sense of "individuality".