Getting rid of a swimming pool

Sorry, but I am very confused by some answers. Perhaps someone can explain it clearly.

In the thread I started about tenant land usage, Tenant Space Usage - #2 by KurtKelley, the replies indicated that the tenant had legal use of the complete space allocated to him or her through the space regulations required by governing authority.

IF a tenant puts up or has a pool or anything else for that matter even though you don’t like it, on the space that (s)he is leasing from you and they are complying with all local laws and have home owner or renters insurance how can you as the landlord deny them the use of that item? Or enforce its removal? Doesn’t your position or rule come in conflict with superiority of law and thus can not be enforced legally? Can’t the tenant then create serious legal problems for you?