Hewitt, the 2002 Wimbledon champion, is second in the world rankings but organisers handed the number two seeding to world number four Andy Roddick on Wednesday because of the American's better recent form on grass.
"I think it's a real slap in the face," Cash, who won Wimbledon in 1987, told BBC radio.
"I don't think it's fair that they should be changing it around."
As second seed, Roddick, who lost in last year's final to Roger Federer, cannot meet the Swiss top seed until the final of this year's tournament. Third seed Hewitt could face Federer in the semi-finals.
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"I don't like them tinkering with the seeds. The guys work the whole year to get rankings to win the grand slams.
"It has certainly given Roddick a boost that I don't think he deserves over Hewitt.
"The rankings are there for a reason. They are fair."
Unlike the other grand slam events, Wimbledon has traditionally varied its seedings from the official rankings in order to reflect players' strengths on grass.
Big-serving Roddick completed a hat-trick of Stella Artois titles on grass on Sunday. Hewitt lost in the quarter-finals.