According to the Memory Alpha article on Starfleet Uniform Code, it is explained that there seem to be some inconsistencies regarding Bajoran earings. Although, as the OP points out, Worf was able to wear his baldric, this may have been because it denotes the symbol of a Klingon's house, according to this article, which is highly significant to a Klingon. (Note I'm not saying that the Bajoran earing isn't important to Bajorans, but it may have been due to cultural understandings between the Federation and Bajorans at the time not being fully developed; indeed apparently one Bajoran serving on the Enterprise-E wore an earing according to the Memory Alpha article which suggests to me that there may have been improved cultural understandings between the Bajorans and Federation, especially following the events of Star Trek: Deep Space 9).
To explain why Riker said this, I do point out that Starfleet Uniform code does specify that:
elaborate headbands and jewelry were in violation of the dress code
hence, perhaps we can conclude that Riker saw Ro's earing as an 'elaborate piece of jewelry' so on that basis asked her to remove it.
Similarly, in VOY 'Learning Curve', Tuvok requires that Gerron removes his Bajoran earing as well. This suggests that the earing is indeed seen as an 'elaborate piece of jewelry', like Chell's Kazleti pendant. This probably reflects Starfleet's lack of understanding of Bajoran customs. Considering the Federation sets up on Deep Space Nine in 2370 and the USS Voyager is lost in the Delta Quadrant in 2371, over a year Starfleet may have not had time to update their protocols to accept Bajoran earings.