That's not entirely true, I've seen Kaito songs about death too. And I've seen other vocaloids sing about almost anything that fits in the rainbow ice cream picture (think of unicorns and strawberry's). It's not about what the vocaloids sing, it's what the producers make them. If you want, you can purchase Kaito's voicebank (or any other voicebank, you could also you UTAU which is free mostof the time) and produce a song about death by ice cream for all I care.
Though you're indeed more likely to find a Kaito song about ice cream than any other vocaloid.