No, of course not. However, if my son wanted a car I wouldn't see anything wrong with telling him that I would buy him a 2-door red Ford if he wanted it. I'm not obligated to buy him any car. If I don't approve of some sorts of cars, I'm not going to pay for those. So rather than giving him money that he might choose to put toward a car I wouldn't approve of, I might instead offer to purchase a specific car for him. If he didn't want it, he could always save his own money to buy something different. I still see that as a generous offer - I could just tell him to save up and pay for his own car. Instead, I would be giving him another option which he could choose to either accept or decline.
I don't see anything wrong with someone offering to get something specific for a couple who was going to be married, whether that's a specific dress or paying for a specific venue or baker or anything else like that. I do see something wrong with someone offering $5000 toward wedding expenses and then later saying they'll only pay if that money is spent on specific things - but I don't see anything wrong with offering to buy a specific thing as long as you never give the recipient the impression that they can count on that money for whatever they choose.
I get the impression, though, that you think it should be all or nothing. If someone wanted to get a car or was getting married it would be rude for me to offer to buy a specific car or pay for a specific wedding expense - I ought to either just give them money or not, but not offer something specific. I guess I just don't get this view. I think any offer if a nice thing. If it works out to be something the recipient wants, great! If not, they can just graciously decline and figure out how to pay for what they want themselves.
(And again, it's not something I would do. But I think that the person whose money it is has every right to offer it for a specific item rather than just handing over the cash if that's what they want to do. It's their money; they can do what they want with it.)