I don't see it that way. You can say it's a marriage of convenience, last resort for women who can't get husbands until their late age. Rather than finish off life alone and without children... or worse fate, prostitution, this could be more dignified for them. And it's within legal framework. Furthermore, it is really up to the woman's consent as is conventional marriage... on other hand, I would say it required mutual consent.

Maybe a bit unethical, but halal. Personally, I'd rather go condemn forced marriages rather than this.