Yes, this is legal and has become a huge business for companies around the world. Despite these trends, there are huge red flags in this form of marketing for companies as these 'users' will most likely not become 'live'/'real' users and will not engage with the page's content. Most companies looking for likes on Facebook will want the users to be interested with their content rather than just for the number. Engagement activity is key!

Hope this helps. I have been able to grow engagement activity for many Social Media pages and would love to help you!