If, by the "BATF doesn't really care" you are asking if the BATF has people scouring online photographs looking for guns that should have serial numbers that aren't showing up clearly in pictures, then, no, they don't care.
On the other hand, if you mean to say that there's no way to end up in prison for having a gun with an altered/removed serial number, then yes, they care a lot, and so should you. Not because everyone who has a gun with an altered serial number ends up in jail any more than every murderer ends up in jail or every rapist ends up in jail. But because if you do end up coming to the attention of the authorities, your life could take a truly impressive change for the worse.