Are business owners contracted to work for AF Legal

If AF Legal buys a firm are the vendors usually contracted to work afterwards for AF Legal for a number of years?
AND
Do they sign non-compete agreements so they can't set up down the road?
AND
If AF Legal buys half a firm like it did recently with Withnalls what arrangments are there for the time when the vendor wants to sell their remaining half share? If they are fair the vendor will have an incentive to build up the business. If not, they won't be so interested

The answers to these questions have caused other roll ups to come unstuck. When the vendors leave at the end of their contract, they take their earning potential, their reputation and their clients and contacts with them

Anyone know or better still point me to an answer?

cheers