I think the issue is, what does "constitutional" mean?

Does it mean "agrees with what I interpret the constitution to mean" or "agrees with what the constitutional court interprets it to mean"? This law is unconstitutional in the first sense, constitutional in the second.

This is not unique to Spain – the US Supreme Court has a long history of interpreting the US constitution to mean a lot of things which aren't obviously in the original meaning of the text. Its recent conservative turn has seen it overturn some of those precedents, but many of them still stand.

Spain's constitutional court – much like the US Supreme Court – is a politicised body – if one doesn't agree with its jurisprudence, the answer is to vote for parties who will appoint judges with different jurisprudence.