Why can I watch a current series on SD but if I miss an episode and want to catch up, I have to have HD?
Not necessarily; The SD versions may be created after entry of the more common HD versions. It might take a little longer before they come out.
The show is available to watch the day after it first airs with SD but later than that, it's only available in HD unless you want to wait and watch it the next time it airs, like at 2 am or something. For instance, I'm watching the current season of Below Deck which airs on Tuesday night on Bravo. If I miss it, I can watch it Wednesday for free in SD. If I try to watch it Thursday, I have to have HD or I can't watch it until it airs at some ungodly hour or just prior to the new show the following week. Really dumb.