Why is it that "They don't like you all" isn't accepted? Since 'was' is the declination of 'wy'
Yes, it's plural.
Actually, the emphasis mostly goes at the end of the sentence. This sentence is rather neutral. "Oni nie lubią was" would be like "They do not like YOU". We strongly discourage such a word order. On the other hand, the word order that feels most natural is "Nie lubią was", with the subject pronoun omitted. In such an order, there's no other place for "was", so the fact that it's at the end doesn't show any emphasis.