It's all in the balance, I guess.
On a very high-volume site, the scoring of each incoming mail -- which requires examining content and evaluating it against what could be a complex ruleset -- presents a potentially significant drain on resources. If a reasonable judgment is made that a TLD is a source of no significant non-spam, then it's far more efficient to just block on the TLD.
It's certainly not uncommon for people or organizations to say "if you want to communicate with me you need to do so in a way that is acceptable to me". The requirements could mean (in descending level of complexity) a local set of rules, or not being on the spamhaus black list, or not using an undesired TLD.
Olivier's issue of bounce messages might be appropriate ... if the recipient of the bounce messages cared at all. I imagine most spamming sites would just drop them.
Arguably that "drastic" action -- cutting off access from a whole TLD -- provides a market-based incentive for that TLD to clean up its act. If enough of the world won't accept mail from a TLD, theoretically its sales would drop and there would be a financial incentive to fix that.
In the absence of any regulatory enforcement of abuse complaints, this is as effective an agent of change as one can hope for.
Universal Acceptance is ICANN's begging the world to live with the products of its TLD expansion, no matter how awful they may be. But given ICANN's lack of any real end-user protections (led by identifiable Board members who believe that end-users are not legitimate stakeholders), this is really the only tool available with which to fight back.
- Evan