I discovered the hard way PBS doesn't have on-demand. I wanted to watch The Vietnam War but it was only available via Internet. I had to install the PBS app on my Roku...get an Activation Code...go to PBS.org...enter said Code...and it worked. Oh, I also entered a bogus email address. I don't want to hear from PBS.
PBS seems to move programs from Available-to-Everyone to Available-to-Contributors-Only. I think it's automatic. If you frequently watch a series, they'll automatically add the prerequisite to force you to donate. Just tough it out and they'll eventually remove it. It took me almost 6 months to watch a miniseries.