This is confusing: I know my two other computers can see each other's tabs and it makes no sense for this one not to have them (and forgot that this one had a different setting). It would even be fine if the UI was there to get to the page and then the page said that I need to enable tabs syncing. That might also help new users get into it.
This was intentionally implemented by bug 517821
As a general rule, disabling a feature should disable the feature. It's enabled by default, so if users choose to disable the feature, we assume they know what they're doing. I do understand the "I disabled it, and then forgot I disabled it" case you're citing, but I don't think it's a majority case, or even a significant minority, for users who disable the pref. This was a subpar experience before, as it's useless UI if the user doesn't want it, so marking this WONTFIX.
Why can't it key off there being any open tabs stored in my account? That would solve the problem for all cases, it seems. (It also doesn't seem hard to envision a scenario where I intentionally don't sync my work tabs to the cloud, but would still like to see tabs from my private tabs on my work PC.)
Because disabling tab sync means we don't ping the server looking for things. Sync is not designed to support unidirectional sync (the same argument can be applied to bookmarks/history/passwords/etc) and we don't really want to go there. If you want to separate work from personal, use separate accounts.