It is not uncommon for parents to get a lawyer when they feel police suspect them. Innocent parents have ended up convicted and in prison.Many parents have been in the same situation, and they have been heavily suspected, even more so than Terri, and they have not given up and stopped talking. They have done the hard work, they have kept the faith and no matter how devastated and panicked they were, they knew that since they knew their child better than anyone, they HAD TO KEEP TALKING TO LE. Because what if, after three and a half weeks, they REMEMBERED SOMETHING that might be helpful to LE, or to other searchers, or to the public, something helpful to find their child?
Innocent parents can't see the future. They do not know for a fact that they will remain a suspect forever. They work to prove their innocence, so that LE can move on from them. They want the perp caught, so that their child can be found. They don't just give up!
Unless they're guilty.