Plus, kids don't magically become better players just because they go to a private school. Many of those kids are much more serious about academics than they are about their musical development.
Finally: these kids aren't going to private school *for band* - I don't know a single "band" private school that exists. I don't even know any private music-focused academies. I know tons of "classical" schools that teach a liberal arts based curriculum, and more that are faith-based - but not a single one set up to get kids a performance scholarship at a conservatory.
Charters and magnets still receive funding from the state - either from the local ISD in the case of the magnet or as a separate entity (charter). They're public schools. A magnet truly draws from an ENTIRE ISD (and realistically also from surrounding ISDs if it's a well-known or prestigious magnet) and draws off the best of the kids, but a charter is open enrollment just like a private school.
You can say "they can recruit from anywhere" - and yes, they can - but the fact is, people don't usually drive across town to attend a charter school unless there are reasons preventing them from attending their local public school.
Do I personally have an issue with the number of private school students who make the ATSSB All-State band? You bet I do. With the rules as currently written, do they have that opportunity and should they take it? Yes, and yes. However, it's literally the same issue that sees small suburban schools dominate the 4A level competition at the ATSSB level. And unless ATSSB wants to simply disenfranchise a quarter of its schools and call itself "Association of Texas Bands From Schools In Small And Medium Towns But Only If They Aren't Suburbs" it won't change - and "ATBFSISAMTBOITAS" is a bit of a mouthful.