Using technology to steal signs is banned in baseball. Teams consider this type of sign stealing a way of gaining an unfair advantage (Joe Maddon compared it to using PEDs). As far as I can tell, though, the reason stealing signs provides an unfair advantage is because they are banned and therefore most teams don't do it.
This case seems very different from PEDs, which have reason to be banned besides the competitive advantage gained. The significant negative health effects from using them are good reason to ban them to prevent them from becoming necessary to perform competitively. On the other hand, nobody is hurt from sign-stealing.
So if the MLB has a sign-stealing problem, why doesn't it just allow it entirely. That takes away the unfair competitive edge from stealing signs.