I reckon your parents reacted like they have because:
When I was told for eg "No, your not to go and see that girl again, your to stay in your room" I would do all things possible to get out the house and see that girl, even leaving pillows under the duvet in trying to trick my Mum into thinking I was still there. Maybe they know they cant keep you to do as they say.
They went out of their way to show gratitude and praise for when you do right, rather than bollock you when you do wrong, no offence, but that is basic dog training yet very very effective and a correct thing to do.
Maybe they just wanted a chilled weekend themselves and have perhaps re-kindled their love for each other and have been shagging all weekend themselves so deep down are too loved up to care. Not a very nice thought for you to hold Funky but us parents still have our wild sides. I was clubbing till 4 on Saturday morning, woke up pissed, sweated Cider all afternoon taking wickets, we can still do it, well at least I can.
Maybe Mum trusts you didn't do anything and Dad dosen't but they have made an agreement not to fall out with each other over this.
Hopefully not this final reason, they have more serious things to deal with right now which you know nothing about; yet.