4 Color Black/Grey Text--Offset Printing

Registration is not the only reason but it is a big one.  Black text with any color out of register no longer appears totally black, wherewith colored type there is less of a visual change because the colored type is usually lighter.  The other more important reason is that large areas od 4 solid colors (large black type areas) can cause trapping issues.  Ink is made to stick to paper, and it does that well.  Ink trapping is how much ink sticks to the previous solid color in the laydown order.  Some presses run Yellow down first and black last. if black doesn't trap well to three solid ink areas then what was meant to look black might very well print brown.   

Yes, the same perils apply to coloured text. However, if they were to enforce a rule to limit coloured text to one plate, you'd only have a choice of four colours for text, and that would be limiting. Printers know that the great bulk of body text and all small text will be in black. If you typeset very small text in colour, you can expect it to be unreadable. This is a particular problem with CD case designs.