I seem to recall that ETA holds little support among the basques, who were quite happy with their autonomy until ETA started to mess things up and Spanish goverment hardened their grip on Basque Land. But how separated "ETA" is, i say, are there many subgroups inside it or is ETA just another umbrella organization รก la Al Quaeda?
If performed by ETA, such a terror strike could backfire on Spanish goverment quite badly. If they harden their grip on the regime, the population propably starts to support more aggressive means to get rid of "too much goverment control".
But this seems to be not ETAish type of strike. Normally they have attacked against political and juristical targets, like judges, police officers and politicians. Attacking this large amount of civilians looks quite weird IMHO.
Oh, and the first one to say "**** THEY'RE ALL TERRISTS NUKE EM TO HELL" will face the wrath of animated .gifs [not derail].