Because... they were drowned... it was the exception to the rule :P
New question: In 1e, were Theans who were known to have sorcerous abilities celebrated, reviled or both? Lots of fantasy RPGs with magically imbued characters treat such people as not a big deal, but I imagine in a setting like 7th Sea, folks with magical powers are 1) Much more rare, and 2) More commonly distrusted or feared by the general populace.
Both. Depends on religious preference and what you knew about the underlining events of the world. Not going into spoilers, but those that followed the church were against magic as one of their core beliefs. Montanigne was open about using Porte magic and their users were celebrated to a certain extent. Many Vaccacians feared the Fate Witches with good reason. Avaloanian Glamour mages were generally accepted loved by their people.
So, it depends on a lot of things. Basically, it was over all neutral with a lean towards dislike, but with big enough areas of support that it was probably possible to have a campaign and not meet someone that disliked you for your magic. They disliked you for another reason.
Sort of. In 1E sorcery was limited to people from noble bloodlines, so they are comparatively rare. It doesn't mean you have to play an aristocrat though. Illegitimate children are quite common, and they can inherit the gift.
Yes, theoretically Sorcery was pretty rare. In fact, on one of the added bits of flavor I liked to sprinkle into my campaigns was that despite the fact that the Montaigne Court seemed absolutely filthy with Porte mages, the truth was that most of the nobility was simply dying their hands red because being a sorcerer was a sign of power and fashionable (after all, Leon is a Porte Sorcerer.) It left some players with quite the surprise when they got themselves stuck in a dead end and turned to the Montaigne NPC they were trying to save from the Inquisition and said "So isn't there anything you can do with that magic portal sorcery of yours?" to which he looked surprised and replied "What? I'm not a sorcerer; this is just red dye."
Yes. It functions like a genetically recessive trait amongst the nobility, generally. If I was asked to put them in order of absolute population I'd probably say:
Porte > Pyeryem > Glamour > Laerdom > Sorte
However, that's still enough Porte sorcerors for the to put a Porte navigator on most, if not every, ship, have a strategic effect on military land communications and have two separate private businesses run by them operate out of Frieburg.