For me its not that these are 'professional footballers and should perform.'
Its more about Pride, Walking out on the pitch I would want to give everything and to win every game. Sure if the opposition is better on the day, whilst I maybe disappointed at least I know I would have given the game the respect and effort.
Lapses in concentration and silly mistakes shows a lack of concentration and desire.
I've played sports and at times I've been promoted or relegated with games to spare and I can say even though the season maybe over (in terms of what can be achieved) I've always wanted to win every game I can.
I may not be a professional, but I'd expect the same from someone who is paid to do it.