So I agree that it could be made more clear, but careful spell does say it protects against a saving throw, not throws. If nothing else, the thing that makes me think careful was never attended to work on more than one saving throw, is just how cheap it is.

Just think about the effect a 'persistent' careful would have. It completely changes the nature of spells like web, stinking cloud, sickening radiance, and more. Your allies can now hang out in the area of effect, completely protected, and enemies have to make repeated saves just to attack them. This all for one sorcerer point? Careful is now probably the most powerful metamagic for it's cost.

Now I do wish there were more spells that worked with careful, but I think it's undervalued currently. Hypnotic pattern is one of the best spells not only for it's level but for a good chunk of the game, and taking away one of its few weaknesses (friendly fire) is a huge plus. Careful can also be used with spells like acid stream, grease, watery sphere, and reverse gravity.