They bring up some good points, graeme_p, and so do you. Automattic, as the SafeHarbor compliant host of several thousands of wordpress.com sites is in a position of being involved directly with takedowns and putbacks so their concern is valid, it is probably time consuming. Wordpress.com offers a way to open a site quickly without much knowledge or expense. Clients of wordpress.com may not be the most tech savvy people, they may have ideas they want to share with others without researching as much as more aware webmasters might. That makes them a soft target for takedown claims not in keeping with the spirit or letter of the DMCA.
There are penalties for misuse, the reason you might not hear so much about those may be because in the type of DMCA Claims discussed, the originating party was trying to avoid publicity in the first place and isn't likely to complain as loudly or publicly as the wrongfully accused. When a counter claim is successfully filed, the entity that originated the claim probably hopes no one ever hears more about it.
Another reason that these DMCA Takedown Claims are often successful is because people don't know the law, don't have the deep pockets to have their attorney look into it for them, or maybe never heard of the Fair Use provisions of the law. It is scary to have your content removed and not in your control. It all goes through a third party, so people often don't file counter claims and the unwarranted DMCA demands are met by default.
Just out of curiosity I did a quick search (your mileage may vary) for "How to file a DMCA?" and it turned up "About 5,959,301 results" while a search for "How to counter a DMCA?" returned only "About 1,319,915 results" so maybe less information is freely available to help people faced with a takedown claim - or maybe my wording of the search terms?