A key concern is that BSD isn't viral, so anyone can take BSD Redis and fork it into a commercial offering. If you want to, you can. The Redis trademark prevents anyone but Redis the company from calling their fork "Redis".
A CLA may impact relicencing, it depends on the terms. A simple CLA may only say "I am the owner of the code and I release it under $LICENSE". The current Redis CLA also has a copyright grant, which gives Redis the company greater rights.
“Viral” just means that the license has a “no additional restrictions” clause, not that you can’t make a commercial offering out of it. That’s why GPL and AGPL don’t really solve the problem.
And the problem with the trademark model is that AWS, and especially Microsoft, already have established brand recognition with the people who sign the big SaaS and support contracts. The people who know what a Redis is are just nerds with no money, the real big shots do everything in Microsoft Excel.
A CLA may impact relicencing, it depends on the terms. A simple CLA may only say "I am the owner of the code and I release it under $LICENSE". The current Redis CLA also has a copyright grant, which gives Redis the company greater rights.