The FAA certification requirement for a fuel dump system is a takeoff
weight that is greater than 135% (I think) of the max landing weight.
They don't add the complexity and cost of a dump system unless it is
required for certification.

The 707-123 I flew had a empty weight of 120,000 lbs, max TO weight of
256,000 lbs, 112,000 lbs of fuel, a max landing weight of 190,000
pounds,(135%) had a dump system. A 757-200 at 256,000 lbs carries
83,000 lbs of fuel, max landing weight of 198,000 lbs (130%) does not.
BTW the 757 with 29,000 lbs less fuel has the same range as a 707-100
with the same passenger load.

The 707 system has standpipes that let you dump only to get you down
to max landing weight, leaving in our case about 70,000 lbs of fuel.
That being said, I'd have flown around several hours after dumping, if
there was no immediate emergency , to get as light and non-flamable as
possible before landing. Their gear problem was not an emergency and
I doubt the crew declaired one even though they asked for the
equipment. Emergencies require immediate action. (think fire) BTW a
single engine failure is not classified as an emergency either. The
aircraft is certified to climb at max takeoff weight on a single
engine.

I'm sure the airlines, EPA, and everyone else would rather the fuel be
burnt as normal rather than dumped into the air and sea. In an
emergency, planes that don't have a dump system will do an overweight
landing and and have to do an overweight landing inspection prior to
next flight. Given the choice, it's better to burn it down than do
the inspection.