If it's true that Disney did that strategically, it would mean they'd be attempting to prevent people from using it on the grounds of trademark law, rather than copyright law.

Either that, or it would be a gambit to get stuff flagged and removed by automated systems illegitimately.

It was used as a way to extend the image to the full amount before reaching public domain. There's some stipulations where items can reach public domain before 100 years, but 100 years is the maximum time allowed. If I recall correctly, they even tried to get public domain removed for this image but it was shot down.