A single non-tackle is not enough of a reason to trade away a guy who is arguably the best at his position. Punters in the NFL are sort of like the interns in an office; no one really pays them the respect they deserve until they're gone. Punters can make or break a game, especially when it's a tight defensive contest and field position becomes critical. Andy Lee may be considered one of the best ever. Cleveland does not have a very good offense, so they are going to find themselves in a lot of tight, defensively-decided games, and they're going to miss Lee.
Yes, the Browns made a mistake when they traded Andy Lee, their Pro Bowl Punter. While a punter does not often get the credit he deserves, he is invaluable when it comes to a team's field position. A good punter, like Andy Lee, can influence the game by positioning the opponent in a difficult place on the field.
No, the Browns did not make a mistake by trading their Pro Bowl punter Andy Lee. This is a business, and in business change must happen. Players are being traded constantly. Sometimes it's good, sometimes more adjustments are needed. But all in all, trade is good and it keeps things fresh.
Andy Lee is a Pro Bowl punter and punters can have a big impact on the game in terms of field position. Still, it's possible that his non-tackle made a difference to the Browns, and maybe they wanted someone grittier. Maybe a gritter player will help the Browns. It's also possible that he'd have been willing to make a tackle during the regular season and his trade might fire him up so he plays extra well for his new team.
Andy Brown has won three times - excellent. He is also ageing, and returning to his home state - and possibly planning for retirement, and wanting to live a life surrounded by his family. The most important thing is that Brown is happy - happy players play better and are less likely to resent the team.