Do ISPs worry about relaying TLS traffic because if the encryption is ever broken, it will be revealed that they were relaying CSAM? There isn't any difference to an i2p node relaying encrypted data and and ISP doing it.

ISPs are, especially in the wake of mergers in many countries, large corporations with lawyers. An I2P node might be run by an individual who lacks that security, and this is just something that can be used against that person if he comes to the attention of the authorities for whatever reason. It’s like the risks of individuals’ running Tor exit nodes, which are well known.

It's the same risk as running a Tor exit node: you getting arrested, having all of your electronics confiscated and facing life ruining charges because someone was caught downloading illegal content from IP addresses that point to your servers.

Even if charges get dropped, or you win in court, that's quite a burden.

Does any transmission method where the message is even partially concealed from the messenger not spark the same concern? E.g. Postal services.

I am pretty sure not blaming the messenger for the message is a legal notion that predates Hammurabi's code.

There is a big difference between storing CSAM (Freenet) and transiently transferring it (I2P).