*Since* in this context means *after* (i.e. - *after* some time explicitly or implicitly identified in supporting text). It might help OP to imagine replacing *since* with *after the time at which* in both his examples.

In other contexts, *since* can mean *because*, but that's really just a metaphoric extension of the above.

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I think what's bothering OP is that *since* has many subtly different usages. Here are just a few from OED:

> 1: During the period between (a specified time) and now; at some time subsequent to or after.  
 2: Ever or continuously from (a specified time, etc.) till now.  
 3: Denoting a point of time to which the action or event mentioned is subsequent.  
 4: In sentences implying continuity of action or fact during the period indicated. Also with ever, and (rarely) with that.

Most (but not all) senses imply continuity from whatever time in the past is indicated by *since*, up to *now*. For example, *"I have known John **since** we were children"* is a very common context, and we normally use past perfect there because *we still know him now*.

But suppose you were at John's funeral? It would be perfectly grammatical to say *"I **knew** John since we were children"* (you don't really *know* him any more, since he's dead). I suspect many native speakers might have the same misgivings as OP, but I think it's just because we're more used to contexts where there *is* continuity into the present.