Why do we say "He who sups with the devil should have a long spoon"?

Meaning:

If you choose to keep bad company you best be very cautious.

Background:

This expression was in use in the 14th century and can be found in "The Squire's Tale", one of Chaucer's Canterbury Tales (c1386).

Who kan sey bet than he? who kan do werse?
Whan he hath al wel seyd, thanne hath he doon;
Therfore bihoveth hire a ful long spoon
That shal ete with a feend,' thus herde I seye.

Which, according to librarius.com, translates broadly as:

Who can say better than he, who can do worse?
When he had well said, all his good was done.
It well behooves him take a lengthy spoon
Who eats with devils,' so I've heard folk say.

It was also in common use in the 16th century, for example in Shakespeare's The Tempest Stephano says of Caliban, "This is a devil, and no monster; I will leave him; I have no long spoon."

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