"Can't be excused?" Hardly. The reuse of a stock music library was entirely normal for 1970s TV animation, and it had been standard practice in movie serials from the 1930s-50s and in many TV series in the '50s and '60s, as well as daytime soap operas and cartoons for decades thereafter. Even the live-action Star Trek did original scores for maybe a third of its episodes and recycled stock music from those episodes for the rest (along with some cues specifically created as generic library music). It's still standard in present-day television in countries such as Japan for a TV series to have a stock library of recycled cues; indeed, many Japanese action shows use the same song or instrumental cue almost every week as a signature theme for a character's or giant robot's transformation, a finishing fight move, or the like.
Really, the only reason American TV stopped using stock music libraries is because the composers' and musicians' unions fought for rules changes that would require new music to be composed and performed for every episode, so that they could stay regularly employed. But that didn't come about until the late '80s, around the time of ST:TNG. In the '60s-'80s, shows were allowed to use stock music for part of each season, but they couldn't reuse a previous season's music in the current season unless they hired musicians to re-record it -- which is why the first-season TOS cues that are reused in season 3 are slightly different performances of the same music, and why the theme arrangement changes every year.
Personally, I love TAS's music, but then, I grew up with it and the same composers' scores to other 1970s and early 1980s Filmation shows. (Which were often mixed and matched, with cues from one show being reused in others. Some of TAS's music originated in Lassie's Rescue Rangers (whose cues were also used heavily in Shazam! and The Secrets of Isis), and the music created for TAS was heard in numerous later Filmation shows including The New Adventures of Batman and Jason of Star Command.