I'm sure being a staunch slaver works if there are plenty of other characters that won't ostracize you for taking that position. I imagine this was the case in ye olde days when the Amarrian RP player base was larger and more unified. By the time I got to the game, a week before Empyrian Age, that was apparently dwindling though. Originally I wanted to play a liberal because I thought it would be a nice contrast to the majority of Amarrian characters since the PF was so extreme, but it turned out that by the time I had quit most Amarrians were as you say. And I thought only Aldy would be like that!
However, I've thought about this phenomenon before, and I think it could be reasonably explained as a realistic outcome of how capsuleering and the Empire work. First off, consider how Imperial society would view capsuleering and cloning. Holders have to give up their titles due to Sacred Flesh, immortality and eternal youth seems to go against nature and God, then couple that with the inherent lack of morality the vast majority capsuleers exhibit and you can assume that Imperial society would not look well upon the position. One would likely have to be more open-minded (or ruthlessly pragmatic) enough to even consider becoming one. Then consider that only rich and well-educated individuals are likely to become capsuleers in the first place. Both education and affluence afford people a wider array of choices when considering world-views, making the pool of capsuleer candidates also be more likely to be liberal than the Empire at large. Then, once an individual becomes a capsuleer, consider the sudden and absolute freedom they then have, both in social interaction and lifestyle choices. They are no longer constrained by Imperial laws about blasphemy and loyalty, and can interact with whomever they wish, wherever they wish. That in itself would be a draw for liberal-minded Imperial citizens looking for an out and would also end up opening the minds of more conservative Amarrians who ended up signing on to a capsuleer program.
These factors, to me at least, make is seem like it would actually be perfectly normal so many Amarrian capsuleers would be so liberal. Also, consider that being a capsuleer is a lot like being in high-school. Popularity for a capsuleer is arguably equivalent to power, as many things are dictated on how many other capsuleers you can sway to do things as you see it. Sucking up to a foreigner or two is not just a pleasant way to pass the time, it is a powerful tool used to gain real influence. A diplomatic and charismatic liberal Amarrian will thrive where a stodgy old battle-axe of a slaver will find himself alone and scorned, except by other stodgy old battle-axes. This is a problem when you want to take down a POS somewhere.
That said, I do find it unfortunate that there are less conservative Amarrians as my time trying to be a liberal lark in PIE was some of the most fun I've ever had in RP, especially when the battle-axe Mitara fell in love with it! The conflict caused truly is fun to play out, and I've also had a desire to play the other side as well. They also technically have it right as the Evelopedia article on slavery states: "Rare is the individual Amarr who desires the end of slavery in the Empire and these are typically ostracized at best, declared heretics as the norm." However, it seems to be the natural progression of things at the moment.