Well, can't say that I blame them. There's definitely money to be had there. Very few of the Disney Animation Studios productions have been sequels, but there are some. Usually the sequels are many, many years apart, Rescuers/Down Under, for example. As long as they have good people on it and make a quality film then I have no problem with it.
Yeah, I wasn't counting the direct-to-video sequels, most of which aren't worth mentioning. They also were not made by the Feature Animation division, but DisneyToon studios. Fantasia 2000 is kind of a sequel, plus there are two Winnie the Pooh movies the came MANY years apart. Also, one might say that Three Caballeros is a sequel to Saludos Amigos, even though both are package films of shorts. They do use the same characters.
For Frozen 2 I'm a bit confused, I don't see what else they could do really. It seems interesting though