Upvote:4
ESTA is one of the statutory requirements for using the visa waiver program; it is specified at 8 USC 1187(a)(11).
The previous paragraph, 8 USC 1187(a)(10), requires VWP carriers to transmit data to the government before bringing the paassenger. The system they use to do that includes a check of ESTA status, so getting to the US without ESTA will be virtually impossible in practice.
The fact that those arriving at the land border are exempt from ESTA appears to be regulated by omitting the mention of land borders in 8 CFR 217.5.