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That's wrong, they don't have to do it. But it sure does help if they're not otherwise getting enough to eat!

https://en.wikipedia.org/wiki/Mantis#Sexual_cannibalism

Apparently it happens more often under laboratory conditions than in the wild.

> Early researchers thought that because copulatory movement is controlled by a ganglion in the abdomen, not the head, removal of the male's head was a reproductive strategy by females to enhance fertilization while obtaining sustenance. Later, this behavior appeared to be an artifact of intrusive laboratory observation. Whether the behavior is natural in the field or also the result of distractions caused by the human observer remains controversial.

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