Actually, its not being clever, its being cheap. If you use
the input frequency to drive the output frequency, you don't
need an extra crystal oscillator. Its cheaper, and results
in fewer customer service calls about the clock running fast or
slow. It also means, you can't just take a 50/Hz european UPS
and plug it into an US plug at 220vac, and expect to still get
50/Hz out of it. When you plug it into the US grid, you'll get
60/Hz out.
I spent a day troubleshooting a problem, until someone explained
to me what was happening. That's why I now use a bench power
supply, which explicitily generates the type of "international"
power I want, instead of using a Rube Goldburg setup.
Most UPS's pass line voltage straight through
unless it disappears or goes out of range. It's
more efficient and results in less wear and tear
than running Ac->DC->AC all the time. You can
usually tell when they are generating power by
the neat buzzing noises they make.
So they all have an ocsillator for when they need
to generate power, but otherwise
what goes in is what comes out (voltage and especially
frequency).
A 240V/50Hz ups will probably generate the desired
240V/50Hz signal only when in battery only mode
(or plugged into a 50Hz line source). Check the
specs to be sure.
KL
Sean Donelan wrote:
Some do both (or their spec sheets lie). I was about
to recommend a unit to Randy, until I downloaded the
PDF and thought 'how the hell do you set the frequency'.
Further investigation revealed something very like:
Input frequency: 47Hz-53Hz, or 57Hz-63Hz
Output frequency: 50Hz +/- 0.1%, 60Hz +/- 0.1%, autosensing
I guess that has a crystal with two dividers, a PLL,
and no obvious way to turn the autosense off.
Most units have some reasonably accurate oscillator
or it makes regenerating the output hard when there
is no input to sync to 
Some good ones have a PLL to
minimize cutover disruption if you take the electronics
out of band.