Because, the typical life cycle of a current USB FLASH drive is way longer than the typical life cycle of an FreeBSD install. There is no serious reason to 'save' it from writing.
Of course, all my special-purpose flash based systems are read-only (for the OS part), but this is because I make extra effort for these to be replaced, not upgraded. For application servers, including my own workstation, I see no reason to make it's boot volume read only. Especially with ZFS, where the chance of corrupted filesystem is much less significant then it is with UFS.
One reason to write on a flash drive is during upgrades. On a typical not-so-fast USB FLASH drive an 'make installworld' may take many, many minutes, sometimes hours. This is because it has to write lots of small files. During this time, if anything breaks, your system may end up unusable. You may sort of speed it up with mounting async for the duration of the upgrade, but not significantly.
With ZFS, it writes as fast as the flash part can and you are safe from crashes, both because of the nature of ZFS and also because you may use snapshots on your boot/root filesystem and revert back at any time, should something fail. Also, I think using ZFS on top of flash will wear the flash part less, because of the aggregate writes (remember, when you write to flash based storage, you have to rewrite an entire flash block each time -- this is why it is so slow).