I just don't think hot ingredients mix with the plastic cup very well, plus I've heard horror stories that the heat makes the cup expand and blow up. (staceyhomemaker.com)
This has to be done very slowly as you never want to mix cold and hot ingredients too quick — the temperatures need to be blended in slowly. (pureella.com)
There's no transferring hot ingredients from one place to another. (wetheparents.org)