Actually, it's udevd itself doing that. A regular child-killing rampage is built into its main loop. To avoid it, one must arrange to not run the desired program directly, but rather to make udevd activate another service that in turn runs the desired program.

To fix this, perhaps it'll need to fork and detach the session, as per the standard daemonisation sequence. c.f. http://www.itp.uzh.ch/~dpotter/howto/daemonize

systemd's session killing stuff will happily kill anything that's been daemonised by the standard approach :(