This has always been the case and it is due to technical constraints not because Sonos doesn’t “allow” it.  Speakers used in surround mode are mere “slaves” of the main HT device and connect to the network differently.

There is no incentive for Sonos to prevent a line in connection if it were possible.

As I understand it the dedicated 5Ghz link from a Sonos soundbar to the Sub or surrounds is not supposed to work in a two way fashion. Sonos would possibly need to rework their surround technical concept to make this possible, and lose sale for the Port or extra Fives.

All the network links are bidirectional. The 5GHz satellites connection demands low packet jitter, to make the low latency operation possible. Putting non home theatre traffic onto the satellite connection could jeopardise this, leading to dropouts.

A counter-argument to that, of course, is that if you’re playing music from a device via the Five’s line-in, you’re not listening to HT needing surround speakers. (Or, that could be made a limitation.) So, if the 5GHz connection is bi-directional it should theoretically be possible. But with no supporting software (or hardware?) for it in the Five, it’s then for Sonos to decide if it’s possible or desirable. 

Not necessarily.  Remember, the line-in can be a source for any and all rooms. So you could be watching TV in one room, and someone in another room is listening to the line-in.  I imagine that would wreak havoc on the low latency surrounds.

^ This

For the same reason it’s not permitted to have a satellite as the sole wired device. The traffic demands of players in the rest of the system could impact the performance of the HT setup.