There is risk in signing him long-term and there's also risk in franchising him. What if he has another 10-15 sack season, what's the price then?
The franchise tag will go up to nearly $20 million, is that his new per year asking price?
He has more leverage in getting a long-term deal if he knows how restrictive placing a 2nd tag on him is.
To me, the concern is not his on-field performance, the concern is his health and I guess his character as far as suspensions for PED's, weed, or whatever. The team has to make their own judgment call on those two things, those are not areas where any of us have much insight.
On the field though he put up 8 sacks his 2nd year, made an impact after breaking his foot as a rookie, and just put up a DPOY season. His 3rd season was obviously a disappointment, but that was because of his health and the suspension.
Knowing what I know of him on the field, I take the risk on a 15-16 per year deal which is more or less market value, and if he remains a consistent 8-12 sack type of guy then it's a bargain.