In theory, yes that's a big part of the problem. In practice, however, once the gov charges you, you're effectively guilty-until-proven-innocent because your court-appointed public defender is likely not going to be trained or equipped to provide a logical defense, much less hire an expert witness in computer forensics. Plus the gov will approach you with a plea "deal" : you can plead guilty to one charge of illegal encrypted data, pay $20k and 2 years' probation, or else risk going to the slammer for decades on the stacked charges with a maximum sentence of 3 years per file, times the 10 files they were "unable to decrypt" on your system.

> arrest me, scan my file system and find something named "plan.txt" which is just a bunch of gibberish... what do you do?

well, start by scanning every executable binary on your system. If they find a custom-rolled program that doesn't impregnate the encrypted files with known headers (for contrast, openssl ads the prefix "Salted_" to any file it encrypts) they can allege that you're using a clandestine encryption scheme and that "plan.txt" is one of the files. So again, the burden of proof would be on you to explain what that file was for, which can come at tremendous legal cost.