Why did they lump all the candidates into one race? That doesn't tell us jack when it comes to two separate primaries.
This is how it will appear on the ballot, and it's actually probably slightly more accurate this way. If instead they had asked the respondents which party they were supporting, then which candidate, a few might have been Republicans for Sanders or Democrats for Trump. This way they can just say "who are you voting for?" and put those numbers down. Undecided also in this case includes people who don't know which party they are voting for, let alone which candidate. Also, the math isn't really that difficult for sorting out the individual primary numbers. From this it's:
Trump - 39%
Cruz - 39%
Kasich - 21%
Clinton - 53%
Sanders - 47%