Yes, probably ideal if they had said, "well, remember you said you're out of town on the 27th? We'll we're going to book that gig, are you sure you can't do it?" Then "okay, well, we're going to get a fill-in for you then". I don't think they need to "ask if it's okay" to use a fill-in. But it's smart if they were to check with their main bass player one more time before proceeding to see if he/she turns out to be available after all. This is especially true because once the sub is booked, it's the sub's gig regardless unless it is some huge deal event; and even then you figure out a way to make it work out for the sub too.
But given that the OP had announced already he could not play that date, it's not a terrible transgression that they booked it with a sub and told him. As far as they were concerned his availability had been checked. It just would have been smarter on their part to see if Main Guy had a schedule change.
If done in the right tone/spirit, and IF the player's schedule is subject to change, and if the player told the band quite a while ago that he was not going to be available (e.g., he didn't just tell the band 2 days ago he is not available), it might be okay to say to the band "hey that's fine, no problem, but next time call me first before you book it, because sometimes my work schedule changes." Of course, if the work thing was pretty much definite and everyone knew it, then I don't see why they need to ask and I don't see that the OP bassist should be concerned that they moved ahead, knowing he wasn't available for sure.