this is pretty common practice for almost any retailer. There are various reasons for it. employee shenanigans is probably one of the bigger ones. If they were to donate clothes instead of having to throw them out and destroy them, then an employee may hide a certain piece of clothing for themselves if they knew they would have to donate it soon.
Also there is the part mentioned above about the brand, H&M and other retailers don't want their clothes to be readily available at the salvation army and have homeless people wearing it, it will then devalue their clothes.
Almost all fast food places will throw out a majority of the food that they can't sell.

I work at a city Landfill & we get very well known stores stuff all the time. I'm unsure what the guidelines are for throwing stuff out or getting it to charity. Even the local thrift shops bring things out here sometimes. Although the items are often in amazing condition, it is illeagal to take anything out of a Landfill once it has entered.