This link: http://www.yandina.com/Soldering.htm goes into detail on how to solder battery cable terminals. Note that he also crimps the terminal before melting in the solder.
If solder will flow through the crimped fitting then you are simply not using a quality crimp tool. With a quality crimp tool (360 degree compression) solder will not flow into the crimp band area. I have taken the lugs I use, filled the end with sugar, made the crimp, then dropped it in a glass of water and let it sit many months (water actually evaporated because I forgot about it). I then drilled a hole and DRY sugar came out.
If you use a low quality tool then yes solder will flow into the crimped area.