thats not what the instructions say. they say the engine block or the negative battery terminal are both exceptable for grounding the unit. they also say that the power wire can be run to the positive battery terminal, starter lug, or any other power junction that is fed directly by the battery except the alternator.
I've wired a ton of these things the same way I have this one wired, and have never had a problem with any of them. this is the first digital box that I have done, and have read that they have been known to give trouble.