I never noticed this, but after thinking about it for a few minutes it seems that in standard usage there is a difference between attributive adjectives and predicative adjectives.
As an attributive adjective, "foot" is indeed correct, e.g. "a 130 foot yacht."
As a predicative adjective, "feet" is correct, e.g. "the yacht was 130 feet long."
Occasionally, in certain regional variations, you may hear "foot" used in the second case: "the yacht was 130 foot long."
A nice combination of SF and fantasy in one picture :-)