Because a carrier’s data on you is not your person or belongings. The companies holding this data are selling access to it, so it’s not being searched, it’s being offered.
In other words, the same reason as why they don’t need a search warrant if there’s a breaking and the business across the street volunteers their security camera footage, even if you’re on that footage.
Are they? It literally points out “Who you voted for is secret” on the ad, right above where it says that people will know if you voted.