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Enforce is the wrong word to use here, because while the idea may have began with the US government, formally speaking the decision to participate or not rested with each National Olympic Committee. The US, and other Western governments in general, simply persuaded (pressured) their respective NOCs into supporting their foreign policy.
Thus the USOC was addressed by Vice-President Mondale on behalf of President Carter, in his capacity as honorary President of the USOC, at its meeting on 12 April, 1980, when by a vote of 1,604 to 797 the decision was taken not to participate in the games.
Siekmann, R. C. R. "The Boycott of the 1980 Olympic games and DΓ©tente." Essays on Human Rights in the Helsinki Process. TMC Asser Instituut, The Hague Google Scholar (1985).
USOC [had] decided not to participate because the President of the United States had declared that the national security of the country was being threatened by international events.
Siekmann, Robert CR. "International sports boycotts: sport, law and politics." Introduction to International and European Sports Law. TMC Asser Press, 2012. 379-419.
That might sound incredible today (after Iraq), but in the context of the Cold War, national security (especially where Russians were involved) was widely perceived to be very compelling reason.
Therefore, the US government did not "enforce" a boycott - the International Olympic Committee did, since (as the Wikipedia page OP linked noted) it would not admit non-NOC sanctioned athletes. Because the USOC was persuaded to agree to a boycott, that excluded US athletes.
Other athletes, sanctioned by their NOC, actually did show up at the Games despite their government's official opposition - including from Puerto Rico, demonstrating that the US government could propose and influence, but did not actually enforce a boycott.
Upvote:6
According to these notes from an International Olympic Committee(IOC) executive meeting, it seems the United States Olympic Committee(USOC) agreed to the boycott mostly on their own due to safety concerns:
After the April 24 session, USOC Exec Dir. Miller said that the USOC explained to the IOC/EC that it had taken its decision after obtaining all information possible. He said there was no question of a saction[sic] and the IOC/EC had not criticized them. USOC Pres. Kane emphasized that the IOC recognized the USOC's efforts to resist political pressure but that the USOC could not have decided otherwise when it came to a question of security. Kane added that if there were to be a spectacular change in the international situation, the USOC could change its stand and send a team to Moscow.
It seems that the IOC acknowledged the USOC's attempts to resist political pressure, which implies that there was at least some pressure by the government to stop the American team from participating.
However, the ultimate reason appears to be security. This was a time when the two countries were very hostile to eachother, so a large number of Americans traveling to Moscow wasn't considered the safest thing to do. The USOC also claims that if the situation de-escalated prior to the games, then the USOC would have changed its stance and sent a team, further indicating it was their decision and not the government's.
The other nations probably had similar reasons, though for many smaller nations political pressure was more likely a factor than safety.