This actually kinda makes sense, and the argument goes like this. It is rather obviously subjective what constitutes "promoting science and useful arts" (including the very definition of "useful" etc). So when it comes to enforcing this clause, you need some body to decide on what these translate to in practice. And this can be either the courts or the legislature. Between the two, the legislature is the one that citizens have more control over, so giving it the ability to decide is intended to make these kinds of abuses easier to rectify (just vote for the right guys!) as opposed to dealing with obstinate courts.