There are two kinds of plans that are billed monthly.
One is an annual plan, billed monthly. Because the annual plan is a commitment for an entire year, cancelling before the year is up incurs a cancellation fee because the annual commitment is being broken. This might be the plan you have, but check the terms you agreed to.
The other kind of plan is a month-to-month commitment. This can be cancelled any month with no fee if the current month is paid for, but it costs more per month.