Whilst this article is behind a paywall, the part visible on the SMH's website indicates the ARLC sought legal advice before this announcement, so I suspect they haven't made the announcement blindly.
After receiving legal advice, league bosses have vowed to impose heavy sanctions on players and agents who hold talks with the breakaway rugby competition.
www.smh.com.au
Further the ban would apply to its competitions only, so the basis of legal action being successful would, I'd suspect, be on the remote side. Also if the player was off contract, it wou;ld mean they wouldn't be able to come back to RL if they find they aren't suited to it. The big ticket item though is the prospective ban on player agents, which I suspect would be a disincentive to agents negotiating with R360 for players under contracxt to an NRL club (e.g. Zac Lomax's).