That would make sense but apparently it was OK'd by the Sox president:
“We like the concept of being the home team in London because we have connections with Liverpool (the Red Sox’s parent company, Fenway Sports Group, also is the parent company of Liverpool FC) and we have a London office so we get certain rights and benefits as the home team over there, our season ticket holders will have the opportunity to get tickets, our sponsors will have access to the games, so we like some of the benefits of being the home team.” According to Kennedy, Red Sox season ticket holders already have shown a strong interest in purchasing tickets to the London contests, which are set for June 29 and June 30, 2019. Kennedy expects both games at the 55,000-seat London Stadium to sell out. “It’s an economic reality that it was just easier to buy out Red Sox home games for the league,” Kennedy added. “So the Yankees have been great. They’ve wanted it to happen as much or even more.”
According to Kennedy, Red Sox season ticket holders already have shown a strong interest in purchasing tickets to the London contests, which are set for June 29 and June 30, 2019. Kennedy expects both games at the 55,000-seat London Stadium to sell out.
So basically it's all about the money.