The American theatre economy can sustain this lengthy period of what is effectively public rehearsal because it has a huge subscriber base. Producers can satisfy the appetite of all their loyal devotees with the previews, then risk the reviews. If they are bad, they have at least made some of their money back; if good, tickets will sell to a wider general audience.
In Britain, shows are allowed up to 21 previews and traditionally, these have always been cut-price. But, with the honourable exception of the RSC, which makes massive reductions, this usually amounts to a couple of quid off, and, particularly if you find yourself watching an early preview, you are effectively subsidising a rehearsal