A lot will make sense when you consider that “sidereal” literally means “stellar.” Nakṣatras are literally stars, “fixed stars” as we might say with contemporary astrological jargon. So, saying that “Nakṣatra are sidereal” is really as self-evident as saying “stars are stellar.”
Rāśi are the “signs” – litterally rāśi means “a bunch [of space].” Rāśī are related to the dynamic interplay of the Sun and Moon as they both move across the ecliptic in relation to the Earth. (That’s why there are 12 of them – there are 12 lunar cycles through the ecliptic during in 1 solar cycle, one year). Rāśi/ signs are not inherently related to stars, they are related to the Earth’s relationship with the ecliptic, measured by solstices and equinoxes (i.e. the signs are “Tropical” – they are anchored on or “the Sun’s north/south movement across the tropical latitudes, creating solstices and equinoxes.”)
So, the signs are tropical and the stars are stellar (sidereal). It seems a radically new idea to many, but its exactly the same as the contemporary concept of “fixed stars” and how those fixed stars relate to the zodiac.
People proposing a sidereal zodiac do so under the conception that the signs and the stars are one and the same, or have a permanent, intrinsic link. This is based on the [now-]classic mistake of forgetting the processional effect on the Earth’s rotational axis (“procession of equinoxes [relative to the stars]”).
The largest user-base for the sidereal zodiac is in Indian/Vedic astrology – but this was not always the case, for the Sūrya Siddhānta and Purāṇas definitely define the 12 rāśi as being anchored to the equinoxes and solstices (with temporary, floating relationship to the fixed stars). Nowadays awareness of this discrepancy between classical and modern Vedic concepts of the zodiac is gaining much better attention, and sooner or later (maybe a century or two from now) the sidereal zodiac will be a thing of the past even in the majority of Vedic astrologers.
– Vic DiCara