Which is actually not always possible. Think about it like this, the shell was tapped with threads once to put the initial threading in. Crossthreading essentially buggered the threading but was essentially running a "bad tap" over the threads a second time. Running a tap a third time to clean everything up just might cause the metal of the raised threads to literally fatigue and give way. The caveat here is most LBS cyclists consider very good bike shops don't actually have the correct tools to retap a BB. In this case you'd want to see a Park BTS-1 or something similar from Campagnolo, VAR, or whomever (depending on English, Italian threading). The Park tool is a $450-600 tool. You'd be surprised how many bike shops with really good reputations don't have the correct tools to do even basic bicycle maintenance. Tools are very expensive, they walk away, and bike frames are more or less considered disposable at this point (carbon paradigm).

Usually there is enough threading to hold the bottom bracket bearings square in the BB shell, but sometimes there just isn't. You don't really know until you try to run a tap through, and even then until you try to install a BB in the cleaned up threads. It can look good but just "let go" when you try to install the BB.

If that happens its not the end of the world. Two tricks every mechanic should have in their toolbox to deal with buggered BB threading, or weird french threading, or even to just get away from Italian threading:

1. Mavic Bottom Brackets - Good bike shops (be advised these are few and fart between) would have the Mavic chamfering tools to install a compression fit Mavic SSC bottom bracket. Think sealed bearings. These are available in a variety of spindle widths if you search on eBay. Absolutely one of the all time best bottom brackets. Think Phil Wood equivalent. Yes they were that good. Strong enough for their 8-speed triple mountain bike group, good enough to win TdF Stages and GC wins (Sean Kelly and Lemond). Its a compression fit BB that permanently chamfers the BB shell. As such many cyclists elected to not use the Mavic BB because they didn't want to damage their vintage Colnagos, Olmos, Yetis or Pinarellos.

2. There are lower quality threadless Bottom Brackets out there. Velo Orange imports one they rebrand as Grand Cru from Taiwan or China. That's an option as well if all the local LBS near the OP are just hack shops without the correct tools to retap the BB or install a quality Mavic BB.