This issue is related to MC-83460
In the issue above it is clearly said that with 1.9 lenient JSON will be gone. Honestly, I hate this change a lot (welcome to backslash hell), but however so far only JSON texts (like in /tellraw or /title or a bok or a sign) can be written in strict JSON.
If JSON has to be written in a strict JSON format in future versions, I want to do this as soon as possible.
If I try the following command, it will not work, although it is strict JSON.

/give @a minecraft:sign 1 0 {"display":{"Name":"JSON"}}

However, it does work like this:

/give @a minecraft:sign 1 0 {display:{Name:"JSON"}}}

or even without any quotes:

/give @a minecraft:sign 1 0 {display:{Name:JSON}}

When I however want to do a /tellraw I need to do strict JSON (as inttended).

/tellraw @a {"text":"I like blue","color":"blue"}

If I want to add this text in JSON format into my book, I need to mix different JSON parsing.

/give @a minecraft:sign 1 0 
{
	display:
	{
		Name:"JSON"
	},
	BlockEntityTag:
	{
		Text1:
		"{
			\"text\":\"I like blue\",
			\"color\":\"blue\"
		}"
	}
}

It becomes even more weird when I now add a clickEvent which gives me a book called JSON that has some JSON-text in it. See:

/give @a minecraft:sign 1 0 
{
	display:
	{
		Name:JSON
	},
	BlockEntityTag:
	{
		Text1:
		"{
			\"text\":\"Wanna have a\",
			\"color\":\"blue\",
			\"clickEvent\":
			{
				\"action\":\"run_command\",
				\"value\":
				\"
					/give @p minecraft:written_book 1 0 
					{
						title:JSON,
						author:me,
						pages:
						[
							\\\"
								{
									\\\\\\\"text\\\\\\\":\\\\\\\"Welcome to the backslash-hell (\\\\\\\\\\\\)\\\\\\\",
									\\\\\\\"color\\\\\\\":\\\\\\\"dark_red\\\\\\\"
								}
							\\\"
						]
					}
				\"
			}
		}",
		Text2:
		"{
			\"text\":\"book?\",
			\"color\":\"blue\"
		}"
	}
}

In the middle of it I have to use for the book's NBT again lenient JSON. An I am not able to use quotes there. This is inconsistent.
I would prefer it much if you could lenient JSON again (seriously look at the amount of backslashes!). But Mojang must have big reasons top change it. But it's a bug that you can't use strict JSON everywhere.
I hope minecraft will move to slash heaven soon again. But let's make it consistent at first. Otherwise the change simply makes no sense at all.