I'd go with "trying to do something nice but didn't think it through". Partly because my DH did the same a few years back, but in my case it worked perfectly
Anecdote which can be skipped at will
I used to be a huge
fan of NKOTB and was actually set to go to a concert with them back when I was 12. Unfortunately I ended up in the hospital instead, and missed out
Fast forward 20 years. I'd just discovered that NKTOB had gotten back together and were touring Europe! Not Denmark though, but they were coming to London. DH knew I wouldn't use that kind of money on myself if he didn't tell me to, so he said he'd give me a ticket for Christmas.
ticket, because I would have gone alone if it had come to that.
What ended up happening was that I called my youngest sister (who absolutely adores
London), and asked her if she'd like a ticket to the concert for her Christmas present, even though it meant she'd have to pay for travelling costs to London. She jumped at the chance, and we ended up having an amazing weekend together.
So yes, my DH gave me a Christmas present that caused me to spend more money, and we in turn gave my sister a Christmas present that caused her
to spend more money.
What puts my DH's actions (and mine, for that matter) in the 'not rude' category is that we talked everything through in advance. We didn't just give my sister what would essentially have been a worthless present, if she hadn't had the money for travelling - we called her up first to make sure she'd be okay with it.
If BIL's doing the same - and not just springing this on people, including his wife - then I think he's in the clear.