Question on millis() and how it disables interrupts

So I'm trying to understand how calling millis() and delay() (which calls micros()) interacts with other interrupts in the system.

unsigned long millis()
    unsigned long m;
    uint8_t oldSREG = SREG;

    // disable interrupts while we read timer0_millis or we might get an
    // inconsistent value (e.g. in the middle of a write to timer0_millis)
    m = timer0_millis;
    SREG = oldSREG;

    return m;

This function calls cli() to disable interrupts, but it never calls sei() to re-enable interrupts. The same happens inside the micros() function. How do interrupts get re-enabled? Is the application that calls millis() or delay() responsible for calling sei() after the function call?

Bit 7 of SREG is the global interrupt bit.

The function stores the value of SREG, disables the bit, then returns the bit to its previous state.