These were dates known WAY in advance that Radcliffe could not perform in. It makes sense to cancel them.
In addition to being required by the AEA union, under the circumstances of this production, an understudy would possibly get to perform if Radcliffe could not perform at very last-minute. Or if he needed to be taken out mid-show.
"The Spectacle has, indeed, an emotional attraction of its own, but, of all the parts, it is the least artistic, and connected least with the art of poetry. For the power of Tragedy, we may be sure, is felt even apart from representation and actors. Besides, the production of spectacular effects depends more on the art of the stage machinist than on that of the poet."
--Aristotle