United was not "scolding" your husband; the agent was simply notifying him that they do not accept liability for checked cardboard boxes and that he should be aware that it's not a proper shipping container.

All airlines require a waiver for boxes, and have been doing so for over 10 years (could be a lot longer, I just can't remember specific examples earlier than around 2005). United's policy definitely has not changed recently; some agents may just be more vocal about explaining why you have to sign "here" and "here" when checking a box.