Answer
Wa `alaykum as-Salamu wa Rahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
With regard to your question, the ruling you have referred to does not make sense as there is no evidence to support it. Otherwise, the players themselves should be prevented from playing with shorts.
In his response to your question, Sheikh Ahmad Kutty, a senior lecturer and an Islamic scholar at the Islamic Institute of Toronto, Ontario, Canada, states:
I would not agree with this ruling for a number of reasons:
1- There is no clear evidence in the authentic sources requiring men to cover their thighs. So, one cannot say that someone wearing shorts is exposing himself, although, I am not in no way suggesting that one should appear in mosques or other public places wearing shorts. Playing soccer, hockey, etc. is a different issue altogether.
2- We have evidence in the sources showing that at least on one occasion a boy who was appointed to lead the Prayer had no clothes except a tattered piece in such a way while prostrating and rising from it, some of his `awrah was visible, and therefore, the Prophet ordered women only to rise from sujud after men have risen and stood up.
The Prophet (peace and blessings be upon him) allowed his beloved wife `A’ishah (may Allah be pleased with her) to watch the dancing with spears conducted by the delegation from Abyssinia in the mosque. It is only reasonable for us to assume that while performing such dances they would not have been fully wrapped up.
In light of these facts, I do not find any strong reason to declare watching this event as unlawful (haram); if it had been haram for women, it would be haram for men as well including the players themselves.
For if the reason for declaring it as haram is that they are exposing `awrah, then how can we allow them to play wearing shorts? How can we allow men to watch them playing?
Therefore, the above ruling does not make sense to me.
Allah Almighty knows best.