Interesting. My understanding was that traditionally BBs were made of ABS, and only bios use PLA. However, that's apocryphal and historical. It might make sense for manufacturers to use a single base material, i.e. PLA. Of course, adulterated with increasing amounts of Mystery Mass: 6mm of ABS would only be about 0.12g, and 6mm of PLA about 0.2g.
Any BB can shatter, and "bio" BBs aren't likely to be worse than ABS for biodegradability (even if the truth is that they take decades or hundreds of years to fully break down in woodland). Since bios work well enough now, and don't cost a huge amount more, I'm OK with following sites' preferences. If they're mandating bios and getting issues with shattering, that's up to them to track and make the call.
Aside, I would flag up that the sites that should be mandating bios are indoors CQB sites. Why? Because they can sweep them up and have them industrially composted, which will actually break them down. Better that than dumping tonnes of ABS (or even PLA) into general landfill.