The developer is subscribing to a "Bait & Switch" mentality. It is free to use but until you pay for it it has a 'watermark' , probably behind the text since the website has no images shown with the 'watermark'. I searched of 15 minutes in vain, looking for the price of the product but was unable to find it without possibly downloading the App.
I constantly wish that developers would be up-front as to the actual cost to turn a demo into a useable product so that I do not waste my time downloading and trying it only to find out the cost is more than I want to pay for it.
So at this point I no longer bother to take the time to download trial or demo version unless the real cost of the product is either in the description easily discovered on the developers web site.
I am of the opinion that if the developer feels that the cost of the product would scare potential buyers off if displayed up-front, then the product is not worth the money!
It is sort of like some ad offering you a electric car for $100 but discovering after you take delivery that it will only go 10 miles on a charge. You then open the glove box and find out it only comes free with a 'trial' battery but costs $80,000 for the full sized battery that will allow the car to go 400 miles on a charge as described in the ad. If you had of known this in advance you would never gone through the hassle of taking delivery of the car in the first place.