Foster has had two seasons worth of OTAs, two camps, and spent all but six days of the 2016 season on the 53-man. That's a lot of reps, and he's still getting reps, at least as part of the PS/scout team. Seems reasonable to assume that the Pats know what they have with him.
How they value Foster vis-a-vis Lewis is an open question for which we'll have to wait for an answer. If Lewis stays a couple more weeks and plays a bigger role than he did in the opener and Foster stays on the PS, we'll be a little closer to that answer.
It's possible that the Pats have been listening to the (alleged) offers for Lewis and simply haven't heard or chosen an acceptable one yet. And I think it's premature to read much into Foster being risked on waivers since they did the same with Karras and, voila!, he's back.