Seems that you're talking about offline isolated converters.
Actually, this is valid for especially Forward, Push-Pull and Bridge converters. The common thing for these is, as you might guess, having an LC output filter (Surprisingly, despite not having an LC output filter, an LLC resonant converter does also need a dummy load!).
If one of these converters with LC output filter works with a proper load which is sufficient to keep the instantaneous inductor current (remembler \$di = 1/L \cdot V_Ldt\$) above zero then this filter can average rectangular waveform coming to its input from the secondary winding. If there's a light load causing instantaneous inductor current to fall zero then the output filter will not perform its job and lets its output voltage to rise as high as its input voltage.
Also, this problem becomes significant on these converters if the control circuitry (which contains oscillator, comparator etc) is supplied from an auxiliary supply derived from a separate winding on power transformer (i.e. supplying converter's control circuitry from directly the converter itself. The control circuitry tries to regulate the main secondary but it's supplied from a secondary which also should be regulated). If the control circuitry is supplied from a separate, free, independent supply then there's a chance to have a stable output even under real no load (RL = \$\infty\$) situation. On-Semi has an application note about real no-load regulation. Note that ATX PSUs are supplied from an independent supply (5VSB).
For Flyback converters, there's something different: Flyback converters generally does not need an LC output filter. However, they do need a dummy load, because they work on something like "store-and-deliver" principle. So, if there's no (or required minimum) load to deliver the stored energy then no stable output shouldn't be expected.
(however none of ATX PSUs seem to have a specified minimal load).
Because all the outputs have a dummy load resistor. Go to this page, open a schematic and take a look at output section. You'll see resistors as dummy loads.