Not buying this explanation personally. Joe is already a mercenary in public perception rather than a hero, and lord knows there are a laundry list of people who would like the S9 obliterated and could conceivably "afford" that revenge. Pretending to be limited to Brockton Bay is not going to make them seem like less of a threat than they already are in any way, nor would waiting for them to come to Brockton have any real benefits over just destroying them as part of some made up contract and posting a video(or doing it in some podunk town the S9 passes through). It's simply an overcomplication in search of justification.The Nine thing is really simple they are far away and he needs to wait until they get closer to avoid people panicking. Since I don't think people truly realise the details I'll explain them as they are now and not from when he first learned of them when they were further away with only a very large an rough area that they were somewhere in and people first called to go out and crush them.
Brockton Bay is in New Hampshire, the exact location is unknown but it is known it is on the very Eastern coast of New Hampshire. From the Nine's current location in the state of New York they have to travel through a fair chunk of the state they are in. No matter the route they take they then have to travel through the entirety of another state and based on their route possibly more than one to even enter New Hampshire. From there they then need to move to the East coast of New Hampshire and go to the Bay itself.
To put it simply that means even now the lowest you can express the distance between them is around 2 and a half states away and may go as high as 4 based on their route. To most probable locations where the Bay could be Google Maps rates it as an around 6 hour drive in modern conditions taking a route which is mainly motorways. However, the nine need to move with stealth in a big trailer to accommodate Crawler so are unlikely to take that route since it has tolls and are in 2011 Earth Bet in an area effected by Bakuda's bombs such as the big EMP one so are likely to need longer.
Going out to crush them now is thus too big a deviation and provokes too strong a reaction to be feasible especially since the nine are in New York state which is Legend's area.