Its not that they don't melt, they just don't ooze into the rest of what it is in. Then, when the cake or cookie or whatever cools, the choc chip regains its shape!
I think that what it is is that they melt after the cake etc has been set so that they melt into the space they are occupying and so can't run anywhere and then set in the same space. Does that make any sense?!! Oh well, I know what I mean anyway!