An offer of free free anti-virus or anti-malware software is essentially a marketing technique
...a way of advertising and enticement to get folks to try a product and if they like it, to purchase the full (or Pro) version which typically provides more features. Marketing and promotional strategies are built into the vendor's business model as part of their operating costs. Bottom line...it's all about generating revenue and finding new and creative ways to do so
. As such, users may have to deal with occasional nagging pop-ups or nuisance advertising and prompts to upgrade to the paid version.By using such free programs, you are essentially agreeing to the terms of the vendor's service which includes those ads.
In some cases you may be able to disable annoying pop-ups through the anti-virus settings if the vendor included an option to disable them. If the vendor does not have such an option listed, then your only alternative is to switch anti-virus software if the pop-up ads annoy you that much.
Also keep in mind that many anti-virus vendors are bundling toolbars and other software with their products as a cost recoup measure and revenue generator. In fact, all free Anti-virus programs now come with toolbars or other bundled software except Bitdefender Free, Sophos Home, Microsoft Security Essentials and Windows 8/10 Defender