Its a wise precaution by Leeds, sometimes you have to make the right call even though it might hurt.

Wider implications may vary - without knowing exactly who the player is and which other teams he or she has come into contact with in the last 2 weeks
then it is difficult to know what the repercussions of this will be.
Best case scenario is it is a player who hasn't played the last two weeks and it is only the Leeds squad who is effected.
Worst case he has played against 2 other teams, it has then got out of control - but could also have got out of control in the first scenario, this is the way with these virus's

For me the league should take proactive steps to understand the spread at this time
Check each squad player for the virus and any squad that is effected should be stood down from games for the next 2 rounds
Immediately suspend all games at anything other than first team level

Those two steps will allow the max number of games to go ahead.

They should repeat the testing on a weekly basis to confirm still clear or now clear

It couldn't be too much of an issue as each club has medical staff and testing 30 players each it could be done in a day

Especially with Toronto deciding to isolate the whole team? Would they get sanctioned!

In the absence of any guidance or leadership from the RFL or Super League, Leeds did the only thing they could do.
Given that it was still a unilateral decision, the process for that still needs to be applied and any sane organisation will conclude that the club took the only appropriate measure available to them. The only question is whether the RFL or Super League are indeed sane organisations.

Should take SL to Court for attempting to send Leeds to France and risking further exposure to Leeds players, Catalans players, match officials, aeroplane passengers and many more. Plus potentially leaving Leeds stranded in France! What sane organisation would try to force that?

Remi Casty has summed the situation up nicely "I'm not sick, I'm not afraid of being either, but I don't want to catch this virus and pass it on to someone more fragile,". He's told the club he's self isolating so not training or playing. Leeds would surely give a similar argument for not travelling

The RFL should have suspended competition earlier anyway - especially any trips through airports and overseas. This is all a load of crap. Leeds have done the competition a favour, if anything they should get extra points for common sense.