NTP does take integrity checking into account [0], though due to export restrictions and regulations the methods have varied over the years and are non-ideal. Specifically, the risk of MITM attacks/abuse is acknowledged as is the risk of DoS via the increased computational overhead of crypto [1].

[0] http://www.ntp.org/ntpfaq/NTP-s-algo-crypt.htm

[1] http://en.m.wikipedia.org/wiki/Network_Time_Protocol#Securit...