I dont think anyone is "allowing them" to cancel. The problem is when the move out date comes and goes and they dont leave. Ive only had one who didnt leave on time but luckily they just needed two extra days. I cant imagine having to do through the eviction process after that.

Our NTV states that once they sign the NTV their apartment is going up for lease. The only way they can cancel is if their apartment is not rented. It is very rare that the apartment is not leased. 

When I put a unit I notice I tell the tenant at the time of notice that once their unit goes on notice that it is available for anyone to apply for and if I get a approved application for the unit they will have to either move out on the date they gave or move to another unit if one is available at the time that once it has been applied for there is nothing I can do so make very sure this is what they want to do.

I also cover myself by stating in the new lease that occupancy cannot begin until the current tenant vacates the unit. It doesn't solve all the issues that can arise, but it prevents the new tenant from having standing. Could result in losing the new tenant if the old tenant stays more than a couple of days

When a resident gives notice, I call and email them to remind them that we will actively pursue finding a new resident to lease their apartment. I also remind them that once their apartment is leased, they will have to be out by their move date with no exceptions. They are reminded that once a lease is signed by the new resident, we are legally bound to the new lease holder and we will pursue eviction against them if they holdover and prevent the new resident from taking possession. Once the policy is made clear, our residents understand and they either cancel their notice to vacate and renew - or - they move out by their move date. Residents appreciate the direct approach and I have never had a resident holdover and prevent a new resident from moving in on schedule. I also send a courtesy email when their apartment is preleased to remind them that they will not be permitted to extend their move date.

I dont have them sign the documents until the actual day and time they are moving in