NASA objects to new megaconstellation, citing risk of “catastrophic collison”
arstechnica.com
arstechnica.com
The cynic in me says NASA has been encouraged to put in an objection by SpaceX who run a competing constellation. Spacex's constellation requires special ground stations, unlike this which can transmit direct to an existing 4G phone. That makes this a big business risk for SpaceX.
I rather not have new players testing the waters of their capabilities with a mega constellation of satellites that won’t deorbit if they fail.
The other part is that there aren't any orbital tow trucks to get rogue or broken satellites out of orbit or out of colliding paths. Plus since you never know where it might happen, and the delta-v would be pretty extreme to go very far or very fast, let alone both which would be needed to prevent a collision of an out of control satellite.
The penalty of screwing up could be large, and there aren't many mitigations available.
I'd guess that's the reason they're at the altitude they are - at lower orbits they'd decay too quickly.
That is just one of the arguments that sound compelling at first read.