Why would they delete them? Just because a file has .old, tmp, etc. as its extension, that does not mean it can or should be deleted by a cleanup program. Much depends on where the file is located.
I'm not sure how you delete files, but the way I approach the task is the same way I use ccleaner for registry :
a) A list of **suggested ** files to be considered for deletion is prepared.
b) User reviews the list, make sure they understand what each one is, and make a conscious decision on each individual file. If none found, press the clean button and recover all that HD space.
CCleaner Registry Tool ... finds and displays all registry entries it recommends for deletion. Includes a checkbox which i can uncheck if i don't want that item deleted.
CCleaner Disk Cleaner Tool ... finds and displays all files it recommends for deletion. Unfortunately, no checkbox which I can uncheck if I don't want that item deleted. Would be nice to be able to add file extensions too.
I don't understand the suggestion that a user should not be in a position to decide what they want found by a cleanup tool and subsequently and deleted from their PCs. Several times a year I encounter instances whereby an AV utility pops up a warning about infected files. This has happened 6 times in the last few years and in each instance they were false positives. The usual tip off was that the say 223 day old file in question was not flagged on Tuesday and on Wednesday it is; and has remained unchanged for 223 days. And when checking file file with virustotal.com ... they all came up clean. But I'm still quite happy that it detects this things because I have the ability to act upon it and decide if I want the suspect file cleaned / deleted or if i want it to be left alone.
Windows by default hides system files but I have the ability to decide to see them. CCleaner doesn't automatically delete files .... it gives me a list of files by category but once found, it doesn't let me see individual files or exclude a found file I don't want to delete. Any backup program I have ever used allows me to include / exclude file types. I don't see why a disk cleaner should not provide a feature which let's the user find file types he / she may not wish to save.