I find your last statement very ironic.
There are valid reasons for using a different server than the ISP provided ones. Whether it‘s latency, as mentioned before deciding who gets to have access to the „valuable personal information“ or simply distrusting the ISP, as any ISP could „unintentionally“ or intentionally do the same as Swisscom has done here. That wouldn‘t colide with privacy laws in any way.
ISP XYZ could say; Well that Website is „dangerous for our users“, let‘s send it to our blackhole / blocking „service“!
-> And then the ISP wonders why users are switching DNS Servers?
Matter of fact! That‘s what it looks like IMP is also (atleast attempting to be) doing. (blocklist.imp.ch)
This is the exact same behaviour as Swisscom in this case.
Getting back on topic, there are many valid reasons. The provider in this case shouldn‘t judge upon this user behaviour. Users are totally free to use their own or public large DNS servers to avoid ISP blocking.