If the seller added this clause way into the proceedings then it sounds to me they didn't know about ir before, its probably their solicitor advising them to add it, or someone else thats been down the same route. Not all clauses are unreasonable, you need to know exactly whats involved before you part with your cash!
We sold a small piece of land to a neighbour, to enable them to have better access and parking to their property, we were fairly friendly with them and agreed an amount, then each instructed our solicitors. Ours advised us to retain the frontage of the land, so that in the event of them selling it for development we still had an 'interest', we decided not to as we were happy with the price, and thought it unfair to our neighbours who only wanted extra parking and as far as we knew were there for life!
2 years later they moved without word to us, the new people put in planning to build a 2 storey eyesore with basement on that land! which is agricultural. (we are in a small rural area of 4 bungalows) Needless to say we objected and it was refused! We're now 'watching this space' as they will no doubt get planning for change of use to extend their domestic curtilage, and then go in for planning again
different story I know but thought it may be relevant!