What everyone seems to be forgetting is that even if another team does think he's worth a first rounder and signs him to an offer sheet, we'd still have the option of matching the offer.
Whereas tendering him at the highest level might have outpriced any team that might want him, and thus prevented any offers whatsoever and forced us to pay him the largest salary (unless, of course we sign him to a long-term deal, in which case, the whole point about his tender is moot anyway), tendering him at the first round level gives us the option to potentially field some offers and ask "Is this guy, at the salary this other team has offered him, more valuable than this team's pick?"
If the answer to that question is yes, then the market has determined his value, we match the offer, and go on our merry way. If the answer is no, then apparently someone else is paying an exorbitant amount for him and/or would give us a very high draft pick straight up for him, which isn't an entirely bad situation.
Let's not forget that, as we all said when Washington traded Bailey for Portis, runningback is one of the easiest positions at which to find a starter.