Why is ITextFont is a CONST in struct IntuiText in intuition.h

The ITextFont seems to be a field that is supposed to be changed, but I get warnings about changing a CONST field. Why is that?

This is the IntuiText structure from intuition.h

struct IntuiText
{
    UBYTE FrontPen, BackPen;    /* the pen numbers for the rendering */
    UBYTE DrawMode;             /* the mode for rendering the text */
    WORD LeftEdge;              /* relative start location for the text */
    WORD TopEdge;               /* relative start location for the text */
    CONST struct TextAttr *ITextFont;   /* if NULL, you accept the default */
    STRPTR IText;               /* pointer to null-terminated text */
    struct IntuiText *NextText; /* pointer to another IntuiText to render */
};