'Rashi': They were converts who converted. David did not want to force Yisrael to do harsh labor [so he accepted many converts]. And so we find with Shlomo below "he counted?; he made 70,000 carriers?"; "ha'Nosar Min ha'Chiti? Lo mi'Yisrael Hemah; ?va'Ya'alem Shlomo l'Mas? ; u'Min Bnei Yisrael Asher Lo Nasan Shlomo la'Avadim li'Melachto" (Divrei ha'Yamim II, 2:16-17, 8:7-9).