Like, why remaster episodes in HD but release them in standard format DVD's?
Because HD players are no longer being made - Blu-Ray won that particular format - but there was still fan demand that the new CGI footage for Seasons Two and Three be released in a format they could play on their regular DVD players. Season One came out as a HD/DVD combo. Now there's no need for combo.
TOS was remastered to keep the syndicators happy about the coming of HD televisions, and still having TOS that would play well on the new technology. 60s TOS FX weren't deemed acceptable for broadcast in HD. Releasing the CGIed episodes on DVD was a bonus.
Just had to jump in here real quick, you're right about the pricing tiers and all that, but there's a bit of a terminology hiccup here. The episodes were remastered in HD meaning High-definition, at least 1080 lines of resolution (vs DVD's max 480 lines). HD DVD and Blu-Ray are each unique formats, both capable of displaying high definition material. I believe the original question was more along the lines of why Paramount isn't taking advantage of BluRay to release TOS-Remastered on to truly show off the new higher definition FX. I myself am waiting for a BluRay release of the remastered eps as I already have the original versions on standard DVD (all 40 of the 2 ep discs at that).