I agree with you. Coming from the zoo field, it's a bit disgruntling to hear about this happening. It really gives Zoo's a bad name. Now I might get some flame from this but I understand the giraffe euthanasia more than I get the lion one.
Zoo's work under a SSP, or species survival plan, and unfortunately the one in America is much different then the one in Europe. Over in Europe, many zoos breed simply because they believe it is healthier for the female to experience gestation periods and motherhood then to deprive them of said experiences. Which I understand to a degree, a primitive animals behavior biologically is to reproduce and ensure the survival of their own. They also believe that contraceptives are dangerous which I also understand. After years of American Zoos putting some of their big cats on contraceptives, we are left with infertile animals once we're ready to use their reproductive skills. Thus killing a good genetic line.
So Marius, the giraffe, came to be simply because of those beliefs. And when it came down to it unfortunately, he was not genetically viable to any European Zoo population, hence the reason why he was put down.
Now when you get to the lion portion, this one I do not understand at all. And that is absolutely senseless, I'll have to read up more on it to understand the logic behind it, if there's any. Why accept a new male lion when you've got two elderly lions that are supposedly "on their last leg" and two "money-making" cubs. That just doesn't make sense. Unfortunately when some people manage a zoo they're more interested in money, reproduction, etc then in the livelihood of the animals they already possess.
Even at my zoo, that I work at, we have an elderly Tiger that's been recently pushed to the side because of the Two new ones my zoo accepted. And many of us are afraid that those in charge will follow similar paths of thought like that of Copenhagen Zoo... new things speak more than old things unfortunately.
EDIT: I should add, that just because I understand the reasoning, in no way means that I agree. That's important to know. Very important...