Since the original production was intended for television and was shot mainly with Panasonic Varicam HD video cameras it makes more sense that it would be interlaced natively. Although the Varicams can do a film-like 24p or 25p, that would require an extra telecine conversion step for interlaced broadcast video use. A Blu-ray at 60i has more information than one at 24p and a Blu-ray cannot output 60p anyway so there is no real need for concern. 24p Blue-ray output is mainly of value for film based content but requires an HD display that can natively refresh at 24 fps or a multiple thereof (not very common except on more recent higher end sets).
Yes, I should have started by saying Frozen Planet Blu-ray is 1080/60i, not 1080/24p. The rest was just pointing out that unless the original source is film there is no great value in having the Blu-ray output 1080/24p. And even if a particular project is film sourced and a Blu-ray is mastered at 1080/24p very, very, very few viewers will have an HD television that can natively display 1080/24p. Almost all are natively 1080/60p. Since no Blu-ray disc can can be mastered at 1080/60p (only 1080/60i and 1080/24p) the player is going to have to convert to a resolution the TV can accept such as 1080/60i or 1080/60p. The point being if it is going to have to be converted to 1080/60p for the TV there is no advantage for the conversion to be made from 1080/24p rather than 1080/60i. So, unless one has a videophile level TV that can display the 24 frames per second natively, there should not be any great concern that any Blu-ray is interlaced or progressive. And to complicate matters, the BBC productions are a special case in that when video based the video is usually shot to the UK standard of 1080/50i. So there is always going to be some conversion whichever way it is converted for the US market.