That's a really good question. Heroku is offering to handle the hosting, devops and scaling issues for us so that we can focus our energy on building a killer product. When considering the costs of hiring a devops team and someone to wear a pager 24/7 when servers go down, paying the premium for using Heroku becomes a lot more attractive.
'why not get a dedicated professional to handle devops on your team'
It sure is easy to write that, but the reality isn't as rosy. I've gone through the process at two companies to try to do that, interviewing ~50-80 people and it has been a nightmare. It is really really difficult to find quality devops people. Again, this makes PaaS like AWS, Heroku and AppEngine a lot more attractive. They are betting their entire business on being able to hire good devops people, so they tend to attract better talent.