Its false hope to be thinking you can avoid the banwave, they dont ban only the accounts that are online at the time.
When you are detected you are flagged, the detection could be just once but its very likely not just once. If blizzard do a banwave and you start an account 2 days later and start botting you are very likely detected straight away. But if they run daily or weekly banwaves, its going to be small numbers of people each time. Its less impressive, people can write it off as being from reports, it doesnt have the shock and awe factor and they cant blog about it to the non-bot players to tell them how they are protecting them.
Also if you get banned and you buy another account, expansions etc and then loose that a week later, do the same a couple of times and are just hitting a continual wall of bans you will stop, if on the other hand you get to think you are getting away with it for several months and you just had bad luck getting caught you will keep buying those games, expansions and subs so they get to milk you for more money than you would have paid in just subs alone
There are other things that likely come into play too, as a publicly traded company they have to realise quarterly updates. If the anti hacking team want to do a banwave I could quite imagine that someone in management will have asked them to hold off, do it a few weeks before the update so that the banned accounts will still be included in the 'active players' totals while the rush of people rebuying accounts will also prop up the 'new subscribers' numbers. Same for a new major patch, those 'new subscribers' mean they can say hey look everyone people are signing up for our new awesome content to temp others to sign up, this is why they will also do discount subs periodically as it helps them massage the subscriber numbers
given that HB numbers are likely waaay down now it probably doesnt have quite as much of difference but with a bit of thought you can absolutely see why they would still continue with waves even if their detection is always on, and detection isnt just about HB
