You can store these on Heroku, but you have to remember that the volumes are volatile - so, you don't really know when given environment will restart and wipe all your temporary files that are not committed to the repo.
If your resources are required, then they should be committed. If you want persistent cache, there are options both inside and outside Heroku.
I think I agree with you though - it's weird for me to have code on Heroku, and then log in to AWS, to put stuff in S3 manually.
