Probably because bash shell option huponexit is set to off (see shopt for option settings).
By the way, disown -h will supress HUP for a specific job, see help disown

In reply to Re^2: Keep a "system" process running if script is prematurely exited? by cdarke
in thread Keep a "system" process running if script is prematurely exited? by westrock2000

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