U.S. FCC rules for loud TV commercials are based on the Commercial Advertisement Loudness Mitigation (CALM) Act. This is written for broadcast stations, cable tv, and satellite tv. Unfortunately it does not apply to streaming tv over the internet, so currently there is no government regulation on this issue for streamers like Roku.
Roku inserts some commercials themselves, but for the most part Roku is just passing along the programming as received from other sources, no different than your TV does.
Some recent Rokus have volume settings you can experiment with under Settings > Audio > Volume modes. See if the Leveling or Night options alleviate the problem.
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