Yes it's a curious feature of JW rules that they have not outright banned JWs from marrying non-JWs. For some other sects this is considered a DFing offence. (the Christadelphians for example, I believe) Especially since research seems to show that religiously divided marriages are very poor at passing on religion. It would seem to be in their own interests for JWs to make Paul's injunction to "marry only in the Lord" a DFing issue.
One way to make sense of the historically liberal attitude of Watchtower on this matter is that JW (women in particular) marrying outside the faith has been a steady source of new members as JW wives often brought their husbands into the religion.
But that approach may now have backfired as husbands are more likely to convince their wives and children out of JWs than the opposite these days. Another example of a feature of JW religion, which was once an asset, or contributed to JW growth, turning around and becoming a major liability.