Folks, you need to know a little bit about this sort of thing to understand the whys and wherefors of how they came to be.
Oh and realise that pirating "skirt bandwidth" is activly in progress today even with relativly modern sat systems.
First off most if not all transponders are going to allow this to happen in one way or another.
Effectivly all a transponder is is an antenna, a filter an optional band shifter, a high dynamic range linear amplifier with 40-80dB of gain and upto 100dB of automatic gain control, another filter and another antenna.
At no point did the signal get actually recieved and re transmited in the conventional sense (ie converted to the base band signal and up again).
There are a lot of similar systems used as "television relay" systems in out of the way places like sparsly populated areas on the other side of hills and mountains.
There are several reasons why these systems are as they are.
Firstly in the design of space systems reliability is very important, as is low weight and low power consumption.
This usually gives rise to extreamly conservative designs with tried and time proven (read old fashioned by launch time) electronics, which is why NASA buys up old ceramic "mil spec" 486 etc components.
Further back in the "good old analog (analogue for us Brits) electronics days" the transponders lack of parts both active and passive and the fact that they required no "re-cal", and therefor had better MTBF figures was highly desirable.
Also they where a known tried and tested technology used a lot in terestrial telecomunications (think multi channel phone links by frequency division multiplexing).
As some will know "availability" is expressed in terms of both MTBF and MTTR.
As MTTR figures for sats is usually considered to be close to infinite ;) the only way to improve availability is by upping MTBF.
The three usuall ways are use only high MTBF parts, use as few parts as possible and operating systems in parallel (ie MTBF goes up as root N of parallel paths).
As both power and weight are significant issues with launch costs and reliability. A lack of parts and parallel systems are highly attractive options to space systems designers of old.
Oh and remember the NSA docs that where released recently where they mentioned long cycle Crypto generators for space systems weighing in at +50lbs and you start to understand why these systems are as they are.
Further and importantly is the 100dB of AGC range, this ensures that the strongest signal(s) always get through the transponder and that it is actually difficult to jam especially when spread spectrum signals are involved.
Some of the sat systems actually had a rudimentry protection system that works extreamly well in that the oscillator used for band shifting was modulated by a non linear long sequence PRBS generator (ie that NSA Crypto generator). Look up JPL ranging codes and Gold generators to see how the linear versions work. Essentialy if you have another identical generator in your transmitter and you shift it's sequence in time to coincide with the one in the transponder then you can use it. The anti jaming margin can give you another 40-100dB of protection over that of the AGC...
However in the past ten years or so the reliability and availability of high dynamic range analog to digital converters with very high conversion rates have changed the equation a lot.
Not just for baseband processing but for high frequency RF processing as well. Have a google around for "channel bank receivers" and "software defined radio" to see the way of the future.
Sats of the future will not be as suceptable to pirating but there will always be those that know or can find out enough to make it possible (think the students that worked out the spreading codes for the European GPS test system just from monitoring the sat output).