Is it against the law to not register the...

Regarding the registration of the lease contract, it has to be registered. If it is not, then you cannot go to court unless after its registration in accordance with law. At the expiry of the contract, the property can be recovered from the tenant under an executive case and under the supervision of the execution judge. However, and if there re any things need maintenance, the tenant must maintain them, otherwise it is possible to claim them through Court, as the Tenancy Law stipulates:
Article (3)
Lease contracts subject to the provisions of this law shall be concluded in writing. Such contracts and any amendment thereto must be registered with the office within one month from the date of their conclusion. Existing lease contracts at the time of implementing the provisions of this law must be drawn up if they were registered within six months from the date of its enforcement. The rental incident may be proven by all means of proof.
Article (8)
The Committee does not accept claims or disputes arising from an unregistered lease ?
The lease contract may be authenticated. In this case, the notarized contract is an official document enforceable in accordance with the Civil and Commercial Procedures Law. The lessor has the right to recover the leased property upon the expiry of the contract term under the supervision of the execution court judge.
Article (10)
The lessor is obligated to carry out the necessary maintenance of the leased premises. If, after being notified by a registered letter with acknowledgment of receipt, he delays or refrains from carrying out such maintenance, the lessee may carry out the necessary maintenance with his knowledge and collect what he spent deducted from the rent, without prejudice to his right to request a reduction of the rent to the extent of the decrease from rent or the extension of the lease contract, or the termination of the contract with compensation if necessary.