White Chocolate is bad for you because it has more sugar (to make it white and give it a good taste) hence it has almost no cacao. More the sugar in the body, more the damage will be as sugar causes diseases like diabetes and also increases blood pressure.
Though I have to agree with some points in the previous answer, not all are accurate. First, most of us know already that such things as chocolate, whether white, dark, semi-sweet, milk, etc. in abundance is going to be bad for us. Excessive amounts of sugar in a persons diet can lead to obesity which in turn can lead to diabetes, high blood pressure, heart problem, etc, however this is "not" a reason as to why "white chocolate" is bad for you.
To say white chocolate is bad for you is not altogether accurate anyway, even "true" chocolate can be bad for you if eaten in excess. However, white chocolate does not contain the nutrients that true chocolate does ergo, white chocolate in comparison to true chocolate is worse for you.
Yes white chocolate does contain "more" sugar than chocolate, however it is not done to "make it white" or necessarily to make it taste good. Also white chocolate is not "white" but a pale yellow or ivory color instead. White chocolate is a confectionery derivative of chocolate. It commonly consist of Cocoa Butter, Sugar, Milk Solids, & Salt. The melting point of cocao butter, it's primary cocao bean component, is high enough to keep white chocolate solid at room temperature, yet low enough to allow white chocolate to melt in the mouth.
Well let's get to the point of "why" white chocolate would not be as "healthy" for you as conventional chocolate. White chocolate does not contain cocao solids, the primary nutritional constituent of chocolate liquor. During the manufacturing process, the dark-colored solids of the cocao bean are separated from its fatty content (as with milk, semi-sweet, and dark chocolate) but, unlike conventional chocolate, the cocao solids are not later recombined. As a result, white chocolate does not contain the antioxidant properties or many characterizing ingredients of "chocolate". These include, thiamine, riboflavin, theobromine, phenylethylamine, and serotonin. Often, the cocao butter is deodorized to remove its strong taste.
Please note some preparations that may be confused with white chocolate (known as confectioner's coating, summer coating, or the brand name Almond Bark) are made from inexpensive solid or hydrogenated vegetable and animal fats, and are "not" at all derived from cocao. These may actually be "white" (in contrast to white chocolates "ivory" shade) and will lack the cocoa butter's flavor.
Please note, I disagree with any connotation that white chocolate is bad for you merely because it is "white" and/or contains more sugar than conventional chocolate, however with that being said, eating most food, in excess, whether it be white chocolate, conventional chocolate, or even other foods, confections, pastries, dairies, etc, can all be potentially bad. A well balanced diet will keep you healthier, even when including luxuries such as white chocolate or other confections.