A disciplinary panel yesterday hit Gers midfielder Black with an immediate three-game suspension and a fine of £7,500 for gambling on games.
The 28-year-old will also have a seven match ban hanging over him that will be triggered if he commits a similar offence this season. The former Inverness Caley Thistle and Hearts player admitted betting on the club he was registered with not to win on three occasions in a seven-year spell.
But Johnstone claimed the majority of footballers in this country gamble on football and believes the governing body has "opened a can of worms" with their ruling.
He feels the SFA must now go after other offenders in the same manner in order to instil confidence in their disciplinary system.
He said: "Everybody knows many footballers in this country bet on games. So are the SFA going to hit them all with 10-game bans? Of course, if rules have been broken then there has to be a punishment.
"If you are not allowed to bet on football, and you have been caught doing so, then you have to face the consequences."
Johnstone added: "But I still find it staggering Black has been singled out. He has taken the fall for something many of his colleagues are doing. An awful lot of them must be worried just now.
"Is the high street bookmaker responsible for reporting Black now going to do the same with all the other players who are doing exactly the same thing?
"I would like to think that now this precedent has been set all the other players will be taken to task."
Since Black was charged, many within Scottish football claimed they had been unaware of the SFA's zero-tolerance approach to gambling.
Players are prohibited from betting on any football match, anywhere in the world.
And PFA Scotland chief executive Fraser Wishart, who represented Black at yesterday's hearing, echoed calls from Rangers manager Ally McCoist for a debate on the rules.
Wishart said: "One of the things that's important to stress, although Ian Black is in the dock today, is this is not just about football players.
"This includes managers, coaches, directors, chief executives, owners, referees and clubs themselves, if they bet on the outcome of their game.
"So we need to have an open and frank discussion."