Foreword:
Okay, you're looking at this from the wrong angle. The same angle Tesco wan't you to look at it - so well done Tesco, the gimmick is working!
Re-iteration
<clicks knuckles>
Before I begin, I will re-iterate. This 'benefit' is not a benefit. It is a gimmick. Your son and son-in-law will not benefit from this in the long run if they are wanting to build up no claims bonuses.
Scenario
You are insured with Tesco. You add your son to the policy. He is on this policy for, say, 4 years, then decides to take out another policy (Also with Tesco). He takes the 4 years NCD and applies them to the Tesco policy. Three years later, your son thinks 'Why is it other companies are quoting significantly lower than Tesco?' based on the 7 years no claims bonus? Well, this is because it would have been 7 years when in actual fact he only has 3 if he decides to move to another company! In this scenario, your son is missing out on 4 years NCD, wasted years and much higher retention premiums due to the transfer clause!
Summary & Closure
If you cannot understand the above procedure or see beyond the 'excellent deal', then operating a motor vehicle is a very dangerous task for you to be given.
Final Word
Not too sure why I bother sometimes. If one isn't prepared to listen in the first place, it's very tiring having to explain it for the second. What's also irritating, is that the same question will appear again next week. <whistles>.