Actually totally understandable given the economics of the trade publishing business.
First of all, LC-GC and Spectroscopy have the same publisher, and so probably have one mailing list with "checkboxes" identifying the pubs to which you subscribe, so if your address changes for one of them, it will change for all.
Second, they are in the business of selling ads (you are not their customer, you are their product
), and their advertising rates are based on the number of subscribers, which means that they have a strong disincentive to allow you to unsubscribe.