The real reason the it's use is restricted is that many aspects of the books are thin metaphors for the real world. There is a clear allusion here to restrictions in British law (and countries like the US that derived from it) on being forced to testify against yourself. The real world reason for these restrictions is that the power to compel self-incrimination is too easily corrupted. Rowling can restrict Veritaserum use, because such restrictions are entirely normal and unremarkable to the predominantly English speaking audience.