Even if the labour is free, reducing it in one area frees it up to be utilized in other areas...has to do with efficiencies.
As well the 50M can be viewed as an investment in the corp. The presumption being that it will lead to growth.
One other possibility (although a much riskier business move) is that if by announcing to the flock of 6 million strong that they have expended massive funds to spread Jehovah's Watchtower's "Jehovah's word" to save more lives, they are able to get an increase in guilt donations, it would be a successful campaign. For example if 1 million of the 6 million JWs kick in a n extra $100 to step up to the plate in this new printing press expenditure then the corp. will in fact end up with 100 million dollars. That would be a successful campaign. Even if only say 250, 000 of the JWs most affluent kicked in an extra 500 dollars, thinking that they should "help out a little extra", that would translate into an extra 125 million for the corp. With announcements of this type there is alot of gain to be had especially where a cult and it's followers are involved. Problem is it is involves more risk but I suspect in the case of a cult it is a far more calculated one. This of course is hypothetical and the best way to know what their motivations were is to get it from the source otherwise we could speculate all day.