Requiring teaching fellows to attend lecture is a great idea. It should have been done long ago. But many of Harvard's introductory science classes are videotaped so that students not at lecture can watch them at a later time. If teaching fellows have a conflict with the time that the class they are teaching meets--they have another class at the same time, for example--and the class is on video, they should be allowed to teach the class anyway.
The staff assumes that it is easy for professors to get the best TFs available. Professors work long and hard to recruit and maintain a stable of hardworking TFs who teach classes year in, year out. If a particular TF has a conflict with the time a class meets one year, they should not be kicked out if they can watch the classes on tape. It is in the interests of the class's students to have that TF available to them.