Its a tricky one - here in Scotland horseriders have the same access rights as walkers/cyclists. Which is easier here as there are fewer people/less pressure on crowded land. It helps riders notm just have more places to ride but also to protect erosion of original ride routes; as it stops what happens in England. There motorists dont want horses on the roads, farmers dont want them on the fields and councils dont want to stump up for alternatives and put money solely into foot or foot/cycle routes only. Even routes used for years without problems are suddenly remove on a whim - the proposed ban (despite no incidents of problems over 30 years!) on horses on the ex railway line Alban way in Herts being a good example.
But the law is the law and based on the law down south there is NO right to ride on a field without permission, so it is not reasonable to slate someone for enforcing their legal right, only to campaign for the law to be changed or for the council to do something about the dangerous road (eg where we used to live, the single track lane is being considered for being blocked at one end due to rat running) or to create a safe off road route.
Also bear in mind some farming payments are threatened if the grassland is damaged eg by being cut up by hooves. While he might not mind one offs, the trouble is other riders see the one using it and then everyone does. And the gate may be locked to stop it being stolen or the field occupied by travellers or used for fly tipping - certainly that was the main reason for locked gates where we used to be.
Alternativel of course the farmer could just be a miserable so and so, and enjoy being unhelpful. But in the end, south of the border, buying the field is the only way to get to ride in it (altho of course that breaks planning rules too in some eyes!)