"We actually do not have a rule against sign-stealing," MLB Commissioner Rob Manfred told reporters last week. "It has been a part of the game for a very, very long time. To the extent that there was a violation of the rule here, it was a violation by one or the other [team] that involved the use of electronic equipment. It's the electronic equipment that creates the violation.
"I think the rule against electronic equipment has a number of policy reasons behind it, but one of them is we don't want to escalate attempts to figure out what a pitcher is going to throw by introducing electronics into that mix."
Sign stealing is a regular part of baseball, though there are unwritten and written rules in place regarding its practice. Teams will often switch their signs midgame or retaliate if another team is caught stealing signs. The Red Sox were fined because their use of electronic equipment broke an MLB rule.
"Despite this clear regulation, the prevalence of technology, especially the technology used in the replay process, has made it increasingly difficult to monitor appropriate and inappropriate uses of electronic equipment," Manfred said in a statement. "Based on the investigation by my office, I have nonetheless concluded that during the 2017 season the Boston Red Sox violated the regulation quoted above by sending electronic communications from their video replay room to an athletic trainer in the dugout. "
The New York Yankees were not fined for allegedly using YES Network cameras to spy on Red Sox signs, per Joel Sherman of the New York Post. MLB said it found no evidence that took place.