Aside from the programmatic problems, what should be done if you have changed an alpha tag (it won't match the main dB...so it would look "updated" to Sentinel). Or, what if the tag is updated both in RRDB and in Sentinel; which one should be used?
Now, add in that Favorites Lists can be made using 3rd party software, so there may be channels in the FL that aren't in (and were never in) the Main DB...Should they be deleted from the FL? (From Sentinel's POV, it would appear that they had once been in the Main DB but were then deleted from there...which is a valid condition.)
What if a TGID for a channel changes...how should Sentinel determine that it is still the "Same Channel"...base it on the Alpha Tag? But, it isn't unusual for there to be multiple channels in a system with the same alpha tag...
There are just so many "what ifs" and "possible if not probable" exceptions, it just really isn't feasible to try to "sync" favorites lists with the Main dB...it is more likely that such a process would break things (from the user's perspective) as often as not.
Keeping the Avoid in the Main dB is simpler (but there are still exceptions...for example, if a channel's TGID changes, the avoid will probably be lost from the Main dB). And, the ramifications of a bad "sync" for avoids is just that you need to re-avoid the channel...much less than a bad sync on Favorites Lists.